This is a great question as sizing is the most difficult part of a breast augmentation for women. The implant can be placed above or below the muscle depending on the individual needs of each patient. In the United States the most common method is under the muscle, however both methods are great if used in the correct situation.
Placing the breast implant below the muscle helps decrease the risk of capsular contracture (abnormal scar tissue around the breast). It also helps women have a more natural and soft appearance to the breast as the muscle acts as an insulation layer between the skin and breast implant. This will decrease the chance that their will be visible rippling of the breast implant as well.
Placing the implant below the muscle does typically make the implant look slightly smaller as the muscle will give the implant a touch more of a natural slope. It also compresses the implant, which makes the breast look slightly smaller. When sizing I typically encourage my patients to go up one implant size, which is 30 to 50cc to make up for this difference.
I would encourage you to spend some time sizing and discussing your implant choices with your plastics surgeon. Your board certified plastic surgeon can help guide you in finding an implant that will help you achieve your aesthetic goals.