that depends on the stream and the day. generally, in my neck of the woods, a 20 standard dryfly hook is the largest i see.
i tie up size 18 2488 for these and will tie them down to 24 2488. i tie 3 different fly styles (loop wing emergers, sparkle dun type emergers and transitionals. i also tye them with standard bwo dubbing and with brown-olive dubbing.
so that is - 20, 22, 24 and 26 in 3 sizes and 2 colors...that is a ton of flies for one general "hatch" but i carry them in the late summer and fall. ok - so i am a bwo and trico nut. i need help!!!
in the spring - i'm much less crazy - the flies seem to hover around a 20 so i only carry 18, 20 and 22. now that is easy...