The issue of silicone gel implants is largely unimportant regarding sizing. Properly done saline filled implants can look just like silicone gel implants.
Bra cup sizes are a very imprecise way of measuring breast size, but in general, A, B, and C cup size breasts relative to the chest around (which doesn't change in an augmentation) are the normal range and D cup size by definition will be out of proportion.
The degree of naturalness of the result is as important as size for looking proportional.
In my opinion, the only way to fit a breast implant and have it look natural and with at least a subjective knowledge of the size increase is to match a round implant to the width of the breast and choose a forward profile that will increase the breast size (unless the breast is unusually wide or narrow) by 1, 1 1/2, or 2 cup sizes from what the breast looked like to begin.
Pictures, computer imaging, and stuffing bras is not helpful in my experience. Fitting a breast implant and having it look right is more like fitting a dress for you with a dressmaker.