There's a thread somewhere on MTBR forum about this. Remember reading about "short" people who ride 29ers… Apparently there's a woman in the States who is 5ft 2 and she races a 29er as her XC bike, and wins races on it! Personally I'm of the opinion that your inside leg length should be larger than the wheel size, but otherwise no issue. If your inside leg length is shorter than 30", probably best to stick to 26" MTBs, if it's above then fire away… 16" On One should suit nicely as they actually size up the same as their 26" versions do.
Oh, and no, you don't have to be tall and/or fat… Smidge under 5ft 11 here, about 11st 10lb, so hardly a big bloke!