500cc Breast Implants for 5'3" and 118 lbs

Breast implants are designed to fit a womans body frame. The implants are designed according to dimensions. The larger the implant, the larger the circumference of the device. If a woman has a small chest, a large implant will cause lateral chest and axillary bulging. Since the breast sensory nerves are located laterall on the chest, there is a greater likelyhood of nerve injury if too large an implant is placed. Implants that are too large will cause more stretching of the breast over time and may ride high following the operation.
It is for all these reasons, that a responsible plastic surgeon will only place a dimensionally correct implant. As far as a 500cc implant for you, it seems too large, but only a dimensional analysis would give a correct answer.

Size of breasts is an individual matter. Your dimensions are small and the choice of 500cc implants will make you very busty, to be sure. As implants get larger in volume, the diameter increases as well. 500 cc moderate profile implants would probably reach beyond the normal anatomic limits of your breast and make you look unnatural. Although I'm not a huge fan of hi-profile implants (they can look a little to globular in my opinion) they are probably the best choice for you give the dimensions of your chest.
I agree with the change to silicone gel. I'll think you'll get a nice result.

Going up to a 500 cc implant is hard to gauge without an examination .  They are fairly large implants to palce on a small frame.