I would guess 6'6 is about average for most SG's in the NBA. I think the average height is about 6'4 for SG's and most NBA players (especially wings) are around 2-3 inches longer in wingspan than height.
Length is something that helps you be a good defender, but it doesn't make or break your defensive abilities. Jimmy Butler is one of the better defenders in the league, at 2 or 3, and only has a 6'7 wingspan.
Alec Burks freaking blows on defense and he has a 6'10 wingspan.
*Most importantly* I don't see anything in Booker's athletic or physical measurements that hold him back from being whatever player he can be. It's not like we are dealing with a 6'1 SG.