So, how do you determine the size of the fly to fish? Say for the MFR.
By trying a size, noting the result, and then going smaller!
For nymphing, I think a lot of people use an average size (14-16) for their point fly and something smaller (20+) for their dropper. The attached somewhat older hatch chart could also be used as a reference.
P.S. You could also stop in and ask at Beavers Bend Fly Shop!
You can fish really any size trout fly and catch fish. But if you’re trying to “match the hatch” you would need to determine what the fish are eating. If there is a hatch in progress, guesstimate the size of the adults as you see them flying by you, landing on vegetation, or floating downstream. Sometimes people sample insects by turning over rocks. However, this just tells you what kind of bugs are in the river and not necessarily what is actually drifting in the river. Seining foam/bubble lines can give you a better idea of what’s drifting. Choose patterns that match the size/shape/profile of any food sources you find.
I recommend buying a package of several paint strainers. They fit over your net and work great. The white mesh makes finding the bugs easy.
If you are sight fishing a nymph or a dry fly and get a refusal, either your presentation may be dragging, or your fly could be too large. Try downsizing your tippet and chose a smaller fly.
Another good tactic if you’re nymphing is chose one fly that’s an attractor pattern and the other that better matches the naturals. An example could be an egg pattern and a midge (a great winter combo).