I agree with Malcolm's observations that the truly large irons were used to get the fly down in high and fast water. This can be readily seen in Kelson's book when he talks about using smaller flies in cold, clear water than in higher, more colored water. Since we have madoern sinking lines, I don't think we need to fish flies larger than a #1/0 or 2/0.
I find that many time people I meet on the river are surprised at the size of the flies I am using. In winter I don't fish spey flies larger than a #1 1/2 Alec jackson and most of the time I am using speys tied on Alec's #3 hooks. In summer, I often fish low-water featherwings tied on #8 hooks where only 1/2 the shank is used for the total size of the fly.
I strongly suspect that fish hit a very large fly out of aggrevation; however, I have seen fish run in terror from very large flies that simply ignored smaller ones.
I prefer to fish the smallest fly that I think is reasonable for the water clarity, water temp, and water flow.