If you want something bigger than 26" - you could maybe try going for a 650b - you probably won't find anything stock, but a 29er is going to be pushing it I think.

I know I googled 29ers and short people a while ago and there were some people at 5'1 riding them - there's a whole thread of people's 29er build for short people on some website (I think I followed a link off here - so maybe search the forum for more 29er threads)