I completely respect what Book accomplished in college but I just don't see even a short stint as a backup. I am sure he will get in someone's camp, but I doubt he get's drafted unless he shows somethings in the Senior Bowl and combines that he didn't show in college.
If he had anywhere near an NFL arm or special skill as a passer, neither of which he has, he might get some time despite his size. And while people rave about his "mobility," he has nowhere near the mobility of smaller qbs like Wilson and Murray much less the arm and passing chops needed for shorter qbs.
Simply put, what he got away with in college, he isn't going to get away with in the pros. His skills in leadership, effort, poise, etc are enough to overcome his lack of pro level passing skills.