Not sure if I'm remembering correctly, but to be a supermodel - i.e. a proper model, not some page 3 bint - then I believe it's 5'6", but it could have all changed by now. To be a glamour model - i.e. page 3 bint, Penthouse poser, etc - height doesn't matter.
I don't see why there is a ruling that only girls over a certain height can be a supermodel (if the rule still applies) because I know very few people of that height myself and I'm sure I'm not unusual. If the excuse is that 'clothes look better on people who are over 5'6" and a size 10', then I think it's time they started making clothes for the masses instead of trying to encourage all teenage girls to be pukers!