Scale_Selection_to_Area

//Tested with ImageJ 1.53g.
//Scales the selection to match a desired area
//Target area unit is whatever unit the image has in the properties

target_area = 5;
//Get area of selection
Roi.getContainedPoints(xpoints, ypoints);
area_pixel= xpoints.length;

//get pixel size in microns
getPixelSize(unit, pixelWidth, pixelHeight);

//Adjust area to microns instead of pixels
area_microns = area_pixel*pixelWidth*pixelHeight;
//Calculate the scale factor
scale_factor = Math.sqrt(target_area/area_microns);

//scale image
run("Scale... ", "x="+scale_factor+" y="+scale_factor+" centered");