It depends on that you want.

Surgeons choose implant sizes based on many different theories with good results.  Personally, I first select an implant with a width appropriate for your chest, then determine how much fullness you want in the upper part of the breast (similar to the effect you get with a push-up bra).  If you wear a 32something bra (32A), you'll want implants that are about 11cm wide, then chose moderate, moderate-Plus, or high profile impalnts.  That will determine if you use a 175cc, 250cc, or 325cc implant.  So which one is right for you?  It takes a good conversation with a board certified plastic surgeon to get a clear picture of what you want and it is his/her job to make sure you're not doing something foolish!  Good luck!