If you are looking for a bigger horse in a few years, I would look at a draft cross or even a 1/4 draft. There are tons of them being sold every day at auctions or you could even look at the PMU horses. Many of them have some draft or mustang blood in them. I am not sure that I would be comfortable breeding your mare to a draft stud because IMHO, she is too small to support a heathy foal from a big boned horse.
There are certain herds of mustangs roaming the country who have more draft blood in them and tend to be bigger 15hh+ but it is really stockyness and strength that decide what size of horse you can comfortably ride. My dad is 6'3" and averages a little over 200 and he can comfortably ride both of my mustangs and they are 14.2 and 14.3 but average over 1100 lbs and have huge bones and feet.
Here is a site, scroll down and look at the "Confusion" HMA. Those horses (they say) are usually bigger and stockier than others. Utah Wild Horse & Burro Areas