How to decide on implant size?

Hi all, Im about a year post op from meta ( no ul, no vaginectomy). I have a fat filled scrotum, but because I had very little bodyfat to work with my sack is now half empty and together with the surgeon I decided i wanted implants. She didn't have samples of the exact implants that can be placed, but showed me a ring with plastic testicles roughly the same sizes. I have trouble figuring out what I want, and what would feel/look natural. the samples looked quite small to me, but when I see other results size M seems bigger than what I have in mind.. There is enough room for M or possibly bigger. I am trying to decide between S or M.. but how?

Any advice?