I also have a question about mouthpiece size. The closest I have been to playing a trumpet before now was about 10 years ago when I was in band and played the trombone. What is a good size mouthpiece for a trombone player to start out with on a trumpet/cornet. I currently have a 7c but it seems a little small in my opinion.

The 7C trumpet mouthpiece is pretty much "the standard" because it is pretty much "middle of the road" as mouthpieces go. A larger cup (1 1/2 or 3 per the Bach/Blessing measuring system) might provide easier transition for an active trombone player. If there was 10 years between when you last played trombone and started playing trumpet, I wouldn't expect mpc size to be much of an issue unless your lips are of the thicker variety.

I HAVE BEEN PLAYING FOR 4 YEARS AND I JUST SWICHED FROM 7C TO 3C AND IT HAS CHANGED MY SOUND FOR THE BETTER SO I THIMK 3C IS THE WAY TO GO

About three months ago I switched from a 7C to a Monette STC1-B4S, which closely compares to a 3C, and it really helped my pitch centering on all notes, has helped further development of my upper register, and is overall just a lot more comfortable for me to play than a 7C.