I have a medium Yo and a few medium/large SAB's, I am 5'9" and all of them fit me ok. The Billys are nice as I like the dropped top tubes. I dont think I could fit on a large.
Depends on what you like I guess. My '92 medium Yo is fine for me and I'm 6'. That said it has a low front end and high post for that classic early 90s position.