I've just found out how to choose between low, mid or hi tops. I know this might not be too important to many people, but I'm kind of a sneaker freak, and it's HUGE for me, lol.
So here are the steps:
First of all, follow this tutorial to be more familiar with what we have to do.
And then, the only thing we have to do to choose the size of the shoe is select a shoe with the size that we want.
For example, if I want Derek Fisher to wear low tops, I have to assign him the Zoom Kobe IV model (one of the very few low tops in the game). As long as I keep the two squares above the shoe model, he will wear his pshoes. The same thing happens with hi tops. I just have to assign him a hi top model (LeBron 8 for example).
It takes work, but its really easy.
Here you have a list of all the models in the game, with their proper sizes