How Smaller Will Breast Implants Be Once Under the Muscle?

There are reasons to place a breast implant below the muscle and reasons to place it above.  These will be discussed by your plastic surgeon.  480cc is large, particularly with you stature but most women wish they had gone with larger implants when queried a year or so after the operation.  The average implant is around 300cc, though.  480 might put you in the D cup are which is fine if that's what you are after.  Sounds like you might want to discuss size again with your surgeon.

Placing impants submuscularly does not necessarily make the breasts look smaller, it just gives them a more natural look and keeps the implant more supported under the muscle. To determine cup size is very difficult and could depend on the manufacturer of the bra. I woud suggest finding pictures of what you would like to achieve size wise and shape to explain to your plastic surgeon.

Your board certified plastic surgeon shoud be able to determine which size implants to use to fulfill your goals. Once in the operating room sizers may be used to determine actual sizes needed. You may need different volumes to achieve symmetry as well. For more information regarding size and breast augmentation, please check out my website and compare before and after photos of pateints. Good luck.

Very difficult to say what size you will be after surgery and cup size is very different depending on the bra it self. But having said that you will likely be more than one cup size larger with that size.
What you need to do is talk to your plastic surgeon, do the "rice test" (explanation is found on my website) or try out sizers and see what look you are going for.
Go for a look, given that the implant fits your body and they see what bra size you wind up being, no vice versa.