Bra sizes are a difficult subject since the bra industry does not standardize cup size.  Consequently, it is very hard to definitely say that a 300 cc breast implant will change your breast cup from an A cup to a C cup.
As I have mentioned in other question, the best way to figure out what size of breast implant will work for you, and thus answer this question, is to trial breast implant sizes in a bra.  This process incorporates the fact that the natural feeling gummy bear breast implants have a flat back, and as a result, your measurements become very important.   As opposed to regular silicone breast implants which are more round than gummy bear breast implants, your chest wall and sizing then becomes much more about your overall size (height and weight) and specific chest measurements such as breast width, nipple to inframammary fold distance and nipple to sternal notch distance.  Once we have those measurements, the next step is to put you in a sports bra and trial implant sizers so that you chose the size and we have an idea of what you want from your breast augmentation.  From there, using implant size charts we can pick a size and shape the suits your chest measurements and approximates the size you liked when you were trying on breast implants.
The main reason why women have their breast augmentation redone is because they did not like their size and almost always that is because their surgeon picked the size for them.
If you do not know about gummy bear implants, especially Sientra implants which now have the best warranty on the market by far and also have been shown to have the lowest rate of complications associated with them read more here.