diff --git a/.eslintrc.js b/.eslintrc.js
index 13362e76c4e473..03c4fd78385277 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -323,6 +323,7 @@ module.exports = {
 						'FontSizePicker',
 						'FormTokenField',
 						'InputControl',
+						'LetterSpacingControl',
 						'LineHeightControl',
 						'NumberControl',
 						'RangeControl',
@@ -343,13 +344,15 @@ module.exports = {
 							'FormFileUpload should have the `__next40pxDefaultSize` prop to opt-in to the new default size.',
 					},
 					// Temporary rules until all existing components have the `__next40pxDefaultSize` prop.
-					...[ 'SelectControl' ].map( ( componentName ) => ( {
-						// Not strict. Allows pre-existing __next40pxDefaultSize={ false } usage until they are all manually updated.
-						selector: `JSXOpeningElement[name.name="${ componentName }"]:not(:has(JSXAttribute[name.name="__next40pxDefaultSize"])):not(:has(JSXAttribute[name.name="size"]))`,
-						message:
-							componentName +
-							' should have the `__next40pxDefaultSize` prop to opt-in to the new default size.',
-					} ) ),
+					...[ 'SelectControl', 'UnitControl' ].map(
+						( componentName ) => ( {
+							// Not strict. Allows pre-existing __next40pxDefaultSize={ false } usage until they are all manually updated.
+							selector: `JSXOpeningElement[name.name="${ componentName }"]:not(:has(JSXAttribute[name.name="__next40pxDefaultSize"])):not(:has(JSXAttribute[name.name="size"]))`,
+							message:
+								componentName +
+								' should have the `__next40pxDefaultSize` prop to opt-in to the new default size.',
+						} )
+					),
 				],
 			},
 		},
diff --git a/packages/block-editor/src/components/global-styles/dimensions-panel.js b/packages/block-editor/src/components/global-styles/dimensions-panel.js
index 5711e16ffadcb1..241d3bb93d1b13 100644
--- a/packages/block-editor/src/components/global-styles/dimensions-panel.js
+++ b/packages/block-editor/src/components/global-styles/dimensions-panel.js
@@ -476,6 +476,8 @@ export default function DimensionsPanel( {
 				>
 					<HStack alignment="flex-end" justify="flex-start">
 						<UnitControl
+							// TODO: Switch to `true` (40px size) if possible (https://github.com/WordPress/gutenberg/pull/64520#discussion_r1717314262)
+							__next40pxDefaultSize={ false }
 							label={ __( 'Content' ) }
 							labelPosition="top"
 							__unstableInputWidth="80px"
@@ -504,6 +506,8 @@ export default function DimensionsPanel( {
 				>
 					<HStack alignment="flex-end" justify="flex-start">
 						<UnitControl
+							// TODO: Switch to `true` (40px size) if possible
+							__next40pxDefaultSize={ false }
 							label={ __( 'Wide' ) }
 							labelPosition="top"
 							__unstableInputWidth="80px"
@@ -611,6 +615,9 @@ export default function DimensionsPanel( {
 					}
 					className={ clsx( {
 						'tools-panel-item-spacing': showSpacingPresetsControl,
+						'single-column':
+							// If UnitControl is used, should be single-column.
+							! showSpacingPresetsControl && ! isAxialGap,
 					} ) }
 					panelId={ panelId }
 				>
@@ -628,8 +635,8 @@ export default function DimensionsPanel( {
 							/>
 						) : (
 							<UnitControl
+								__next40pxDefaultSize
 								label={ __( 'Block spacing' ) }
-								__unstableInputWidth="80px"
 								min={ 0 }
 								onChange={ setGapValue }
 								units={ units }
diff --git a/packages/block-editor/src/components/letter-spacing-control/README.md b/packages/block-editor/src/components/letter-spacing-control/README.md
index 4288f67a2aa99b..ef119bbc943d2f 100644
--- a/packages/block-editor/src/components/letter-spacing-control/README.md
+++ b/packages/block-editor/src/components/letter-spacing-control/README.md
@@ -38,13 +38,20 @@ The current value of the letter spacing setting.
 
 A callback function invoked when the value is changed.
 
-### `_unstableInputWidth`
+### `__unstableInputWidth`
 
 -   **Type:** `string|number|undefined`
 -   **Default:** `undefined`
 
 Input width to pass through to inner UnitControl. Should be a valid CSS value.
 
+#### `__next40pxDefaultSize`
+
+- **Type:** `boolean`
+- **Default:** `false`
+
+Start opting into the larger default height that will become the default size in a future version.
+
 ## Related components
 
 Block Editor components are components that can be used to compose the UI of your block editor. Thus, they can only be used under a [`BlockEditorProvider`](https://github.com/WordPress/gutenberg/blob/HEAD/packages/block-editor/src/components/provider/README.md) in the components tree.
diff --git a/packages/block-editor/src/components/letter-spacing-control/index.js b/packages/block-editor/src/components/letter-spacing-control/index.js
index 2c36e59cd8241c..1577e184c4a06c 100644
--- a/packages/block-editor/src/components/letter-spacing-control/index.js
+++ b/packages/block-editor/src/components/letter-spacing-control/index.js
@@ -15,14 +15,16 @@ import { useSettings } from '../../components/use-settings';
 /**
  * Control for letter-spacing.
  *
- * @param {Object}                  props                      Component props.
- * @param {string}                  props.value                Currently selected letter-spacing.
- * @param {Function}                props.onChange             Handles change in letter-spacing selection.
- * @param {string|number|undefined} props.__unstableInputWidth Input width to pass through to inner UnitControl. Should be a valid CSS value.
+ * @param {Object}                  props                       Component props.
+ * @param {boolean}                 props.__next40pxDefaultSize Start opting into the larger default height that will become the default size in a future version.
+ * @param {string}                  props.value                 Currently selected letter-spacing.
+ * @param {Function}                props.onChange              Handles change in letter-spacing selection.
+ * @param {string|number|undefined} props.__unstableInputWidth  Input width to pass through to inner UnitControl. Should be a valid CSS value.
  *
  * @return {Element} Letter-spacing control.
  */
 export default function LetterSpacingControl( {
+	__next40pxDefaultSize = false,
 	value,
 	onChange,
 	__unstableInputWidth = '60px',
@@ -35,6 +37,7 @@ export default function LetterSpacingControl( {
 	} );
 	return (
 		<UnitControl
+			__next40pxDefaultSize={ __next40pxDefaultSize }
 			{ ...otherProps }
 			label={ __( 'Letter spacing' ) }
 			value={ value }
diff --git a/packages/block-editor/src/layouts/constrained.js b/packages/block-editor/src/layouts/constrained.js
index b48e331792bd8a..b8dcde637a4d30 100644
--- a/packages/block-editor/src/layouts/constrained.js
+++ b/packages/block-editor/src/layouts/constrained.js
@@ -75,6 +75,8 @@ export default {
 						<div className="block-editor-hooks__layout-controls">
 							<div className="block-editor-hooks__layout-controls-unit">
 								<UnitControl
+									// TODO: Switch to `true` (40px size) if possible (https://github.com/WordPress/gutenberg/pull/64520#discussion_r1717314262)
+									__next40pxDefaultSize={ false }
 									className="block-editor-hooks__layout-controls-unit-input"
 									label={ __( 'Content' ) }
 									labelPosition="top"
@@ -96,6 +98,8 @@ export default {
 							</div>
 							<div className="block-editor-hooks__layout-controls-unit">
 								<UnitControl
+									// TODO: Switch to `true` (40px size) if possible
+									__next40pxDefaultSize={ false }
 									className="block-editor-hooks__layout-controls-unit-input"
 									label={ __( 'Wide' ) }
 									labelPosition="top"
diff --git a/packages/block-library/src/cover/edit/inspector-controls.js b/packages/block-library/src/cover/edit/inspector-controls.js
index eedd94de88e4aa..10bd3de890a0c4 100644
--- a/packages/block-library/src/cover/edit/inspector-controls.js
+++ b/packages/block-library/src/cover/edit/inspector-controls.js
@@ -73,13 +73,13 @@ function CoverHeightInput( {
 
 	return (
 		<UnitControl
-			label={ __( 'Minimum height of cover' ) }
+			__next40pxDefaultSize
+			label={ __( 'Minimum height' ) }
 			id={ inputId }
 			isResetValueOnUnitChange
 			min={ min }
 			onChange={ handleOnChange }
 			onUnitChange={ onUnitChange }
-			__unstableInputWidth="80px"
 			units={ units }
 			value={ computedValue }
 		/>
@@ -299,6 +299,7 @@ export default function CoverInspectorControls( {
 			) }
 			<InspectorControls group="dimensions">
 				<ToolsPanelItem
+					className="single-column"
 					hasValue={ () => !! minHeight }
 					label={ __( 'Minimum height' ) }
 					onDeselect={ () =>
diff --git a/packages/block-library/src/cover/test/edit.js b/packages/block-library/src/cover/test/edit.js
index ab99d3c555e3bc..ca34c15d0b7ec6 100644
--- a/packages/block-library/src/cover/test/edit.js
+++ b/packages/block-library/src/cover/test/edit.js
@@ -372,10 +372,10 @@ describe( 'Cover block', () => {
 					} )
 				);
 				await userEvent.clear(
-					screen.getByLabelText( 'Minimum height of cover' )
+					screen.getByLabelText( 'Minimum height' )
 				);
 				await userEvent.type(
-					screen.getByLabelText( 'Minimum height of cover' ),
+					screen.getByLabelText( 'Minimum height' ),
 					'300'
 				);
 
diff --git a/packages/block-library/src/post-featured-image/dimension-controls.js b/packages/block-library/src/post-featured-image/dimension-controls.js
index fa0f022332b0cb..b31b68b7811976 100644
--- a/packages/block-library/src/post-featured-image/dimension-controls.js
+++ b/packages/block-library/src/post-featured-image/dimension-controls.js
@@ -134,8 +134,7 @@ const DimensionControls = ( {
 				panelId={ clientId }
 			>
 				<SelectControl
-					// TODO: Switch to `true` (40px size) if possible
-					__next40pxDefaultSize={ false }
+					__next40pxDefaultSize
 					__nextHasNoMarginBottom
 					label={ __( 'Aspect ratio' ) }
 					value={ aspectRatio }
@@ -157,6 +156,7 @@ const DimensionControls = ( {
 				panelId={ clientId }
 			>
 				<UnitControl
+					__next40pxDefaultSize
 					label={ __( 'Height' ) }
 					labelPosition="top"
 					value={ height || '' }
@@ -179,6 +179,7 @@ const DimensionControls = ( {
 				panelId={ clientId }
 			>
 				<UnitControl
+					__next40pxDefaultSize
 					label={ __( 'Width' ) }
 					labelPosition="top"
 					value={ width || '' }
diff --git a/test/e2e/specs/editor/blocks/cover.spec.js b/test/e2e/specs/editor/blocks/cover.spec.js
index fa5103ebaa4eeb..cf3005c9eaa18f 100644
--- a/test/e2e/specs/editor/blocks/cover.spec.js
+++ b/test/e2e/specs/editor/blocks/cover.spec.js
@@ -158,7 +158,7 @@ test.describe( 'Cover', () => {
 
 		// Ensure there the default value for the minimum height of cover is undefined.
 		const defaultHeightValue = await coverBlockEditorSettings
-			.getByLabel( 'Minimum height of cover' )
+			.getByLabel( 'Minimum height' )
 			.inputValue();
 		expect( defaultHeightValue ).toBeFalsy();