On my commander I believe it was a 13mm on the gear box side, I had to replace that seal. Those u joints are about 130$ if I recall correctly, and nothing aftermarket is the correct size last I checked. Make sure you use locktite on it, we had problems with the front drive shaft bolt coming out on the gear box side. I believe it's locktite 271 that I used, the permanent stuff.
Last time I checked was over a year and a half ago, when I had my drive shaft extended by a drive line shop, I had them check for me. The mav might be different , but I did some research back then and the quad guys were using something that was close but not quite the same and had some issues with it.