Height isn't really a restriction anymore because of frame designs. Technically, I had no business getting on a 29er according to conventional wisdom, but I talked ot Peter at Misfit and he convinced me that fit wouldn't be an issue for me (I'm 5'6"), and he was right. Someone smaller than me could ride my 29er without much of an issue. At a certain point, I suppose it gets unmanageable, but the same can be said for a 26er (hence, the 24's available for kids.) But in general, unless you are really short (and maybe 5'2" would be the cutoff -- I don't know) a 29er is an option.
The only issue I have with my small frame -- and this seems to be the case with almost every small frame, 26er or 29er -- is that it only has one mount for a cage on the front triangle. I hate to race with a Camelbak - I just can't stand having anything on me when I ride -- but for longer races, I've had to suck it up because even with aid stations, I can't carry enough water with only one cage. If I have to carry food and multiple extra bottles in my pockets, it's no better than having the damn pack on me. I've tried the external mount cages and have had mixed results at best -- 40 miles into a 50 mile race, my seatpost mounted cage from Topeak loosened up and fell into my tire. That sucked. I wish someone would think of a way to better utilize the space available in the front triangle to fit an extra cage. (It isn't going to be me because I'm not terribly bright.)