In reply to Sam_in_Leeds:
The theory is that 29" wheels roll better over uneven ground and carry a bit more momentum (some people claim they climb better but that's just the gearing effect that can be got by shifting), but they are quite a bit heavier (a 29r tends to be a couple of pounds more than the same bike on 26"), and some people (including me) don't get on with them. It's also a bit uncertain how long 29rs will be around, and most bikes seem to be going with 27.5" now.
My mtb is 26", I tried the other sizes, but 27.5 wasn't really around at the time(2 ears ago) and i didn't like the 29rs that were available...they all seemed really slow to react to steering input and sort of rose up on me too much for my liking and i like a manoeuvrable, fun bike rather than an all out xc racer (I go out to have fun, not to ride the fastest I can). I'm 5'8" Generally shorter riders will get on better with smaller wheels, as the frame geometry is a bit compromised by the bigger wheels on smaller bikes.
It really is a personal thing though so try all the sizes before you buy... preferably off road on varied terrain.
Post edited at 19:11