If I see fish rising to bugs, I will try to match the bug to size and shape, especially on unfamiliar water.

On waters that I usually fish, I have a better clue as what to use. For caddis, 9 times out of 10, I'll use a #16 tan EHC.
For BWO's, I'll start with a #18 parachute BWO. If that's not working I will work down in size from there down to a #24.
Rarely will I go up in size....only if I see lots of big bugs.

If I see the tiny midges flying around, I'll use a #22 or #24 dry in various colors to try to match them.
Sometimes I will see other types of mayflies in the air. For those I'll use a #14 to #18.