The "knee high" boots (generally called Tall Boots) are fitted by the shoe size, the height of the boot and the width of the calf.
Tall boots need an almost exact fit. If the boots are too short it's not the appropriate look, too tall and they will bruise and cut the back of your knee open. If the calf is to wide, they won't provide proper support, too narrow and you won't be able to put them on.
Unfortunately, riding boots aren't something that you can pick out for another person because if they don't fit right, they will be excruciatingly miserable. Tall boots aren't comfy to break in when they fit properly, I can't imagine how painful an improperly fitting pair would be.