If I understand it correctly (I'm not a medical professional) the actual size and health of the donor's kidneys, and the relative body mass and surface area of donor and recipient are measured. The relative height, of course, is considered within this but is not the only deciding factor.
Also keep in mind that if because of size difference or other factors a particular donor may not donate to his/her chosen recipient, sometimes a "swap" can be arranged where different sets of donor/recipients interchange kidneys so that every recipient gets the best match. Some of these exchanges can involve a dozen people in different States.
www.kidneyregistryorg has a nation-wide data base and tries to organize such exchanges.
Fr. Pat