There are lots of riders over 6' tall that ride 20" BMX. I feel weird on a 20" bike but I'm 40 and have a lot of injuries. I have a 24" BMX(well I have 2 of them)and it might feel too big as trick bike to someone 5'5". Go try a few different top tube sizes if you can. You can probably find a 20" bike that feels comfortable.