This is a very good and common question. Trying on implants under your bra will not give you a good idea and the newer digital photography systems which will give you a picture of what you might look like can be very deceiving.
The best idea is to meet with your plastic surgeon and ask to see his or her before and after results. You need to find patients who have breasts that are the same shape and size as you and see what they look like. If you like the results then you are probably going to end up very happy. Looking at the photographs can be deceiveing unless you find patients who look like you before surgery.
Breast implants are sized by measuring the base of your breast, base width. Generally, Allergan and Mentor implants will have 3 profile choices in your appropriate base width. The low profile (Allergan) and moderate profile (Mentor) will increase you about 1 cup size, the moderate profile (Allergan) and moderate plus profile (Mentor) will increase you about 2 cup sizes, and the high profile implants will increase you about 3 cup sizes.
In your case, going from a small B to medium C cup would require a low or moderate profile in your base width. One piece of advice is that you can always go bigger but it is much harder to go smaller so if you have any doubts I would advise going with the smaller implant.
