He has 4-5 inches max vertical on Klay. For me his athleticism is not a problem at all. His length could be, but I am not sure and won't be sure until I see him defend in the NBA.
Booker's size is fine, good even, if he can be a combo guard. The key is passing, handling, vision.
FYI, Booker is 1 1/2 inches taller than Brad Beal and has the same standing reach.