Way more about shape for me than size (assuming they're within realistic proportions in relation to the rest of the body). Prefer perky ones over saggy ones and smaller areola over bigger areola for example.
I remember reading a plastic surgery study about how breasts with a 45:55 ratio (of upper to lower breast or something, don't remember the details) are considered the most visually appealing by the majority and while I don't necessarily agree with plastic surgery, the examples of boobs with such ratio did all look fucking great