6' would be fine if you exclude the Bradman factor. Top 7 over 6':At what height would the best "Under <Height> XI" and the best "Over <height> XI" be best matched? 6 foot is a natural height to pick, but with Bradman, Sobers, a host of other top batsman and an attack along the lines of Marshall/Steyn/Larwood/Murali I think the Under-6-foot XI would have a definite edge.
B Richards
Hayden/?
Kallis
Pollock
Chappell
Miller/?
Gilchrist
It's perhaps marginally weaker with the bat than a short 11 (even if you pick a specialist over Miller) but you have more bowlers to choose from.