There isn't a maximum height unless you show pocket or standard. XL males are over 20″ at the withers and females are over 19″ at the withers. I am not sure how tall the tallest Am Bully is (bred to standard and without Mastiff mixed in) but I will find out. I wouldn't imagine any taller than 23".
Bernie is 23 inches tall, the XXL Bullies can get to the height lauren mentioned. Now Bernie is far from show quality but he has a very consistent pedigree on his dad's side and they were all power midgets, not sure where he got the heigh from. Although he has Juan Gotti twice in the ped and I heard he was over 20 inches tall. I would preffer a tall dog if I am dealing with a heavy frame but that is just me.