Well, believe me I am far from the authority on the BFB, since I've been there 4 times over the last month or so and have only one baitfish sized chub to show for it, but this is what I have observed:
Just last Friday I was up in the fly fishing section and seined both clinger and swimmer mayfly nymphs in moderate to fast current. Size 14 was prevalent with some 16's in there as well. Brownish olive was the coloration, making a Pheasant Tail nymph a good choice. I also seined a few stoneflies, both golden and black, in the fastest waters, a crayfish, some good-sized hellgramittes and only a few cased and free caddis. It's no secret, but for matching the aquatic insects, seems like a #14-#16 Phesant Tail should do well.