Thank you for your question. I am assuming that you already had a consultation and the surgeon gave you the implant size option of 400 cc. If so, then they must have done some breast measurements to determine a range of implant sizes that would be suitable for your body type. If you want you can request bra-sizing trials to visualize how your breast may look. Please note that during bra-sizing, the bra adds a bit of volume (e.g., 30 cc), and the process underestimates the final breast size by 15-20% based on studies which compared bra-sizing to 3D imaging. Nevertheless, please note that we want to achieve long-term aesthetic results, and so we should not go too big or too small as you already affirmed. A common pre-operative concern is going too big, but after surgery, a regret is not going big enough. Psychology proves that when you have more options, you feel dissatisfied with your choice. However, if you base your implant size purely on your breast measurements and the desire for long lasting results, then better outcomes are likely. As such, the implant size has to fit your body dimensions. The more you deviate from the proper fit, the more fake your breasts will look. Your surgeon may have used tissue-based planning to determine the implant size. The planning takes into account the effects of implants on tissues over time, risks of excessive stretch, excessive thinning, visible or palpable implant edges, visible traction rippling, ptosis (sagging), and breast tissue wasting. To determine the final implant size, breast measurements such as breast base width, breast skin stretch, and nipple-to-inframammary fold distance. The combination of these measurements will help determined tissue coverage and the required implant volume to optimally fill the breasts. This information may be combined with your preferences and desired goals for a final profile and implant size. Please share all your motivations for surgery, your concerns, your expectations and goals with complete honesty and clarity. This will help prevent post-operative concerns/regrets. If you trust your surgeon and their judgment and the decision-making method used to choose the implant size, then you should be in good hands. Hope this helps.