Alright, so if your mare is 17hh and the jack is about 13hh, you can get a mule anywere between 14 and 16 hh. Getting a tall mule depends on the following factors:
1.) Is it a mare mule or a jack? Jacks are always bigger by one to two hands. If you don't geld you jack, he will be smaller than he would have if you big at a younger age.
2.) Does the jack usually breed large mules? Check with the jacks owner about getting some pictures of the jacks previous foals and see what hight they were in comparison to the mare on average.
Next thing is about the color. The color inheritance of color is the same as if you were breeding your walker to a stallion.
Oh, and the jack need not be gaited. Gainted-ness is dominant in mules, so, unless I am wrong, the foal should be gaited. (I'm not sure about this, so check with some smarter people.)
Please post some pics of the foal when and if you get one! Baby mules are my weakness.
Don't tell the other members, but I think they are ten times better than horses.