Lets face it, the only guys who say size doesn't matter - are those guys who don't have
a big one
I tend to base the size of the fly on the end of my line with light and clarity levels. If it is bright and clear I will downsize somewhat, (as I use tubes length is my only measurement) say 2-3". However, with sunk flies as in spring or winter fishing I am less concerned than I am when summer fishing with a floating line.
I will use outright huge flies. Even before I saw first hand how large some of Ed's Intruders actually were I had used my Voodoo Child and Raging Prawns in lengths up to 7" - with success.
The probable reality of this fly size colour debate is most likely that the only thing that really
matters is that the guy who is fishing actually believes
in the fly he's using. If I don't believe in it I NEVER seem to catch anything. But if it is one I believe in - I KNOW I am about to catch one!
We've all done the 10 casts and switch routine while searching for the "magic" fly - I wasted so much time at that that I decided to throw out all but the 2 or 3 patterns I really believe in. I have them in a range of sizes - but it makes the choice issue real easy.
These debates are fun and often informative, but my sincere advice is to settle on a few patterns you have confidence and fish the hell out of them