Cotton,
Welcome to TM! You'll find a lot of come back players on this site. I am back from a 15+ year trumpet layoff myself. Yes the Bach sizes actually seem to run backwards numerically, but it's a fairly easy system to follow. I have always played a 7c and just recently have begun to branch out. Here is a link to a great resource for comparing MPs to each other and to other manufacturers...
The Ultimate Trumpet Mouthpiece Comparison Chart
In most cases a larger mouthpiece will create a broader sound and tend to be used in orchestral work. Notice that I said 'in most cases', because every player is different. To pick out a MP I would suggest that find the nearest music store that carries a decent selection, try out lots of different MPs and find the sound that you are looking for. I would also suggest that you wait until after you spend some time developing your chops. It took me a good six months to really get back to a decent level to know what I was looking for and I wasted some money buying MPs that were all wrong.
You will probably find some good information by searching the archives for other posts on this topic! hope this helps!