Quarterbacks have little if anything to do with the skin's problems. They need to draft linemen for three years and then worry about QBs. Jason Campbell was perfectly good and I assume Shanahan understood that and that the move for McNab came from Snyder. Rex Grossman is as good as they need until they can get a real line in front of whoever's playing QB.
Quarterbacks, runners, and receivers come from the normal part of the human race as for size, i.e. you've got a lot to pick from. When you go shopping for real athletes who are 6-5+ and over 300 lbs, then you're not talking about the normal part of the human race any more and the air gets a whole lot thinner. High draft picks have to be used for linemen as a general rule.