They give you a choice to try and they give their opinion any risk with certain sizes like rippling or sagging etc if a certain size would increase that risk, they will also refuse certain sizes if they feel a iplabt too small would not fill loose skin or if an implant would be too big, it is their reputation after all and they wouldn’t want to put you at greater risk so it’s a joint decision they tend to go with what you like the look of rather than focusing on a cup size as no 2 implants look the same on everyone as there is back sizes, chest shape, implant size, shape and profile all contributing to the final result xx