Any heavy riders riding shorter horses? I am looking at a 13.3 hand gypsy at 2 years old. He's quite thick buiilt. I wouldn't be riding him till next spring. I am 5'7, 260# and losing. Beginner rider, though I have rode when I was younger.
Height does not matter so much as a nice, strong, short back with a wide loin coupling and good bone. Halflingers and Fjords are common mounts with the plus sized ladies I chat online with. Shorter they are, the shorter the fall, too. ;)
With an appropriate support system, I don't think that young horse + green rider is a complete write off. My instructor and I were just talking last night about how the best components of a relationship with your horse are forged on the ground and sometimes you miss out on those fundamentals when you have something already broke and are itching to get into the saddle. I feel like I 'know' my horses well from spending a significant amount of time on the ground with them.
Drafts and draft crosses are also good choices, but I think each horse really needs to be evaluated as an individual. :) The automatic assumption is that a very large horse can carry a large rider but that is not always true - particularly since full drafts are traditionally bred for pulling, they tend to have longer backs not as good for weight bearing. :)