Thank you for your question. Based on your photograph you have excellent anatomical normal full breasts. Placing the implants underneath the muscle does not make them look smaller. As the implants drop over the first 3-4 months may push the breast forward and look what you would expect with...
Large round bowling balls on your chest will not stay perky but sag with time and thin your tissue. Long term, you will do better with smaller implants that fit your frame and tissue. Otherwise, your second surgery will probably include a lift. Large breasts sitting on your lap do not look good.
I think 350 cc's would be a very nice size for you. However, I suggest you go back to your surgeon and ask to try on sizes again. Please do not choose an implant that is too large and heavy for your frame and tissue. Otherwise, your second surgery will include a lift and probably going smaller.