You know, Imma big girl, and when I first wanted to get into riding I was worried about hurting the horses. I started out on a draft cross, and have been riding QH's.
I am by far an expert, you'll get LOADS of good advice on this site. But it depends on the horse's weight, I've been told that a horse would be comfortable with 20% of it's weight on it's back, including tack. So, you'd want something weighing in upward to 1500 +/-. A horse with a shorter back might be a better choice than one with a longer back, and just over all a nice sturdy build.
I think if I didn't weight a million pounds, I'd still be attracted to the horses that are built like tanks. I like the draft cross I used to ride, but I also like the QH's I've met too. The QH's seem to be a lot smoother ride than the draft x, should keep that in mind if you have a sore back or tough knee's or anything like that.