I prefer a smaller bike to a larger one. My road bikes are all a few sizes too small. I was lucky enough to score a 20.5funi xc frame a few yeara back. Its exactly the right size. I'd go to a shop and sit on a few different sizes you'll soon figure out what feels best. Then you can find a good second hand bike. Too big is really uncomfortable on an mtb, you can't get the vontrol you need for single track.