I think they would be fine. That's not that much difference in size. But minimule is right, she will cycle and will stand for him. Just so you know too, the odds are like 100,000 to 1, but there are documented cases of molly mules having foals. Usually it is a case like this, where people believing they are competely sterile, lets a molly live with a stallion. If the timing and a million other things are right, you never know. Hey, it'd be something to tell us all about!
But, back to the size, in one pasture, I have my 15hh older appy mare and 2 mini jacks. In the other pasture I have 2 mammoth donkey jennets, one standard jennet, and 6 mini jennets. They all get along great and no probs. The old mammoth even likes the little baby jack. She thinks he's hers!