I know everyone says go smaller and smaller, but size 18 and 20 have produced for me in dry, emergers, and nymph patterns. Maybe because by the time 20-24 size was needed I was hitting the ice more and getting skunked on the river.
The larger sized drys and emergers work fine for me in the spring right through the early fall in southern Maine, but at some point in October I always end up downsizing. I prefer to use 20s rather 22s, but sometimes I have tie on a 22 even though they are truly a pain to fish with.