I think we spend too much time worrying and not enough time fishing
I fish #10's, LS #12/14's down to a #20 for wild Browns. They all catch fish. I fished in a gale on a loch in Attadale with a #14 taking fish from the surface while my mate, fifty yards away, was catching them on a #6 Murrough. I was fishing from a point down and across the waves and he was pulling through them. My normal cast is #14 dry (my own pattern) on the top dropper, #12 Iron Blue Dun on the middle and a #14 HillLoch nymph (my own pattern) on the tail. I occasionally change the dry fly for a floating nymph, LS #12/14 (my own pattern) if the fish are taking sub-surface and come up a size, to a #12, on the HillLoch Nymph if there is nothing showing an interest, or vice versa if I start with the #12. The change usually results in takes.
At Moidart, after observations of the water, I put on a #10 Invicta, in place of the Iron Blue Dun, it caught fish but I can't say my catch rate increased dramatically.
To come back to your question, most of my wild browns are caught on #12 or #14's.