I do not prefer a small horse at all. I would say at least 14.2-15hh. It's the horse's knowledge and training that will matter. Get a good solid trained horse for her.
I personally prefer the draft breeds. I use to own 3 of them. I had a PMU mare, never had anyone on her back and I sat my girls on her back and she didn't do anything. Might have been different if I had of gotten on her back. But, drafts are very easy to train and they are very smart. Of course, you can not do things with a draft that you could do with a QH. So, depends on what she has in mind.
I personally prefer the draft breeds. I use to own 3 of them. I had a PMU mare, never had anyone on her back and I sat my girls on her back and she didn't do anything. Might have been different if I had of gotten on her back. But, drafts are very easy to train and they are very smart. Of course, you can not do things with a draft that you could do with a QH. So, depends on what she has in mind.