Ok, I can't do the conversion in my head right off hand, BUT....at 250 pounds I rode a QH pony mix (she barely made it to the horse height requirement to NOT be a pony). Best horse I ever owned and she could and would carry me all day if it was necessary (used her for EVERYTHING....pleasure, working cattle, showing, etc) She was not big boned either, but rather slim legged and on the lean side.
The biggest thing with figuring out what horse to learn on is to find one with a good temperament. I would add training to that, but don't think I've ever owned a well-trained horse (as my nephew asks: How come you get all these crazy horses? Answer: By the time they are in MY price range there HAS to be a reason they are that cheap LOL) By personal experience, I'll also say that shorter horses are way easier to get on and off when you are a heavier person. My step-stool or bucket are seldom far away if I am doing any riding.