Caddis for me mostly 14-16. Mayfly hatches 14-18. Midges 18-22.
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.
I don't fish dries that much, mostly wets or scuds. When I do, it is always a #14 CDC & Elk. Seems to work whether fish are rising or not.
It really depends upon the water I am fishing. With rivers, some of them produce really small bugs, like the Missouri River above Cascade, Montana. That river is usually fished with very small dries, BWO's, Caddis, PMD's all usually no bigger than a 16 but often in 18 and some in 20's. In contrast to that is the Clark Fork and several other western rivers in Montana where larger dries are often used, like 12's and 14's, but then again if the hatch is a smaller insect then you go to a smaller dry.
If I had to go with one main size I would say a size 16 would produce well on most rivers that I fish. That is unless the Skwala Stoneflies are out, then bring out the 12's and 10's. As for other stoneflies, Salmonflies are huge, Yellow Sallies are small.
As for lakes, again, depends upon the water, but 16's are almost always a good bet.
BWO 18, PMD 16, PED 14, March Brown 12-14, Green Drake 8-10, Callibaetis 14, Caddis 12-16 (except October Caddis or Traveling sedge), Salmonfly 4-6, Golden Stone 6-8
Depending on what is hatching, anything from#10 to #30. Generally, the larger sizes in the spring, smaller in the summer, and smallest in the fall, but there are exceptions.
Yup. Most of the fish I caught in October and November were tied on Gamakatsu #28 and #30 hooks.