If I settle on say 500cc, should up size a few cc if going under muscle?
500 cc is quite a large implant. I would make sure that your surgeon fully explains the risk of using a very large implant in a breast augmentation. I would not recommend using an implant this large in a subfascial/subglandular pocket. I would not recommend an implant this large in general. I would go see a board-certified plastic surgeon (with 6 year integrated plastic surgery training and 1 year The Aesthetic Society accredited fellowship).
Implants placed under the muscle may look a little smaller versus those placed subglandularly. There are some benefits to going under the muscle such as disguising the upper portion of the implant and lower occurrence of capsular contracture. Your surgeon will advise you if it is recommended for you to increase your implant size further.