feat(pubsub): set channel size #1997

Job	Run time
cfg-check	1m 19s
docs	1m 53s
clippy	1m 6s
doctest	1m 50s
feature-checks	3m 20s
fmt	11s
no-std	17s
test stable --no-default-features	1m 45s
wasm-unknown	49s
wasm-wasi	1m 28s
test stable	1m 56s
test stable --all-features	2m 20s
test nightly --no-default-features	2m 48s
test nightly	2m 45s
test nightly --all-features	3m 3s
test 1.76 --no-default-features	1m 27s
test 1.76	1m 25s
	29m 42s

Provide feedback