Joe is correct that your horse grows every 20 miles.
I would not at all worry she is going to be too big. I just saw a Freisian/Arabian cross this weekend and a draft/arabian cross at the race I was at and they did quite well, and they are quite a bit larger than the mare you are looking at. It really, really depends on the horse's mind, their conditioning, and whether or not their basic conformation is good enough to prevent back, leg, hoof, or any other soreness issues. Though I will say a horse that gaits tends to have a higher heart rate if conditioning is equal, just because the act of gaiting equals more heat (more work = more heat = higher heart rate ... same with more weight = more heat = higher heart rate). The horse you are looking at has no obvious conformation flaws, so I think you should be fine as long as you condition slowly and methodically.