I think the answer to this question lies beyond fly fishing.
There is a far greater wealth of knowledge accumulated by gear fishers on the topic of what sizes/colors get strikes, and which don't. The fish have no idea that a "fly" or "yarn+corkie" is being thrown at them, they just react.
And if you ask gear fishers, they will tell you without hesitation that size/color does matter. I don't fish using these methods, but I have to agree, and the proof is in the pudding.
that said, without a good presentation, you'll inevitably have a "nice day on the river" at best