Hi Muirenn, I bought the very basic MTB Trek 820 WSD. When I bought the bike they gave me a basic fitting and adjusted the the handlebars and seat. I ended up moving the seat back because I have "ape arms" according to my partner. I kept moving back on the seat and hanging off the back. It's much more comfortable now since I no longer feel the need to push back on the seat, but my right cheek sure is getting sore!

As far as sizes go I'd be surprised if I fit a medium (though I hope to one day) My waste is 32 and my hips are 40.5.