Would love to see pics...
As a general rule, height doesn't matter (IMHO) in regard to how well a horse will move, rather how WELL he is built.
However, if your mare is slightly built, I would not even consider breeding to a stallion more than a hand or so taller than her...the risks to her health, from having a much too large of a baby are too great. Same with a stallion who is really 'broad' in build...Just be careful of what you breed her too, especially because she is on the smaller side.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."