After one year of BE practice and after trying some new mpc's I think I want to play a smaller one. My band director is opposed to this plan and says that it will sound too harsh and brassy, but it plays very comfortable. What should I do?

This can be difficult. If you really play easier on it (with the same or greater range, flexibility endurance and tone) then I would suggest to stick to it, whatever the band director says. A mpc that may sound a little brassy and even harsh in the beginning can smooth out after some time. When your lips adjust and are able to focus better in a smaller cup, the sound can be as warm as it was on a bigger piece. It can take some time, but it is not dangerous to try it, if you play comfortably on it. You can also try to find a compromise between your directors choice and your own, but this can lead to a lot of confusion and frustration. If you play a bigger mpc and your endurance suffers, you play with a beautiful big tone for the first fifteen minutes and after that you still don't sound good..On the other hand, a good mpc is always a compromise. It depends on what you have to play, how long, high, what sound you or your director wants etc., but if you find a mpc that seems to fit well, even if it is a small one which your director hates, give it a chance.