My breast is flat. I want to go as big as a doctor can make. But all I hear is that nobody will make me more than 300ml

I'm sure it is possible to put more than 300 ccs but I'd have to examine you. It's based on the width of your chest, the look you want, and quality of your skin.

When I have a patient who wants more breast volume than I can fit into their frame at the first operation, I advise them they might need a second surgery after nine months. At that time the tissues have been stretched and may allow me to place a larger implant. There is a limit as to what size can fit. The doctor needs to be able to close the incision. Hope this answer helps you and good luck. For more information on this and similar topics, I recommend a plastic surgery Q&A book like "The Scoop On Breasts: A Plastic Surgeon Busts the Myths."

 For long term natural looking breast and to protect  native breast tissue, implant size is limited by your tissue characteristic  like breast width, native breast tissue, skin tightness. if 4 plastic surgeon says for limit you have to accept it, as personal examination is the final judgement.You can go for larger implant after 6 to 9 months afte primary surgery.