To a very great degree,the exact sizes depends upon the water your fishing. Even in the same geographic area, some streams or ponds are more fertile than others. That said, I've found in my area of SW Ohio the following sizes seem to match the average of what I seign from the creek:

January -
mayfly nymphs about size 18
caddis larva about size 18
crane fly larva - up to size 6
helgrammite - up to size 4
shad - 3" long
Stonecats or madtoms - from 3" to 8" long
bluntnose minnows, shiners, etc.. - more than 3" long

April -
mayfly nymphs about size 12
caddis larva about size 14
crane fly larva - up to size 4
helgrammite - up to size 2
shad - 3" long
Stonecats or madtoms - from 3" to 8" long
bluntnose minnows, shiners, etc.. - more than 3" long

July
mayfly nymphs about size 20
caddis larva about size 16
crane fly larva - up to size 4
helgrammite - up to size 2 with more below size 8 than in cold months
shad - 1/2" long or even smaller - size 12
Stonecats or madtoms - from 1" to 8" long
bluntnose minnows, shiners, etc.. - from 1/2" to more than 3" long
Fry - bluegill, crappie, etc... about 1/2" and almost transparent

October
mayfly nymphs about size 18
caddis larva about size 20
crane fly larva - up to size 6
helgrammite - between 8 and 4
shad - 1" to 2" long, some up to 3"
Stonecats or madtoms - from 2" to 5" long
bluntnose minnows, shiners, etc.. - from 3/4" to 2" long
Fry - bluegill, crappie, etc... about 1 1/2" and translucent