IAmBrassman

I agree with most of these posts. My opinion is start with a piece no larger than a 3C and no smaller than a 10 1/2 C. Logic on each solid one in that range:

3C -- as previously stated, good all-around mouthpiece with reasonable depth and diameter. Fairly comfortable rim.
5B -- My symphonic favorite for years. Slightly narrower than the 3C, but actually only slightly deeper than the 7C. Rim on the 5B is similar in comfort to the 3C. The shape of this cup gives a very warm, dark sound.
6C -- The oft overlooked great piece. Everything the 7C is, but with the cup depth slightly shallower and a much more comfortable rim.
7C -- Medium width with a slightly deeper than standard C cup. People with large, or very soft, lips (like me) may find this rim too 'sharp'.
10 1/2 C -- Much closer in size to 'starter mouthpieces' used in the late 1800's than the 7C. Some consider it too small a starting place. I do too, with only one exception -- if you have sharp teeth, very squishy lips, limited air supply due to a physical challenge, or simply cannot get any of the others to work, give this one a try. It's a very forgiving piece, fairly comfortable rim, but significantly smaller cup volume. (Get a good teacher who knows how to teach the smaller pieces before going this small.)