(In our opinion they run on the small side.)
5'8" JP on the size 48cm Haibike SDuro AllMtn 6.0, a great fit.
5'11" Shea on the 48cm Haibike SDuro AllMtn 6.0. This is the size he prefers as he likes to catch big air and whip the bike around, so he likes a little smaller fit. If Shea were going for just fast XC riding, he would probably be on the 52cm.
6'4" Nick on the 52cm Haibike AllMtn 6.0. A good all around fit, although the pic does not convey that as he does not have his foot all the way extended.