flyguys--I haven't read the other postings in reply, and that's intentional. I'll look to see what others think after I respond.
On size, I consider widths of 15-25 feet medium-sized and anything above that a large stream (by Smokies' standards).
As for elevation, the magic number for me is 3500 feet. Above that I consider a stream to be high elevation, and the number is also a pretty good arbitrary figure for where you'll likely begin to find specks. Obviously that's a generalization, but it covers a lot of situations. Jim Casada