As for what men prefer in terms of chest size, women need to understand is that there's an enormously wide range of breasts that men like. Whatever you happen to have is probably pretty great to the eyes (and hands and lips) of most of us.
So no, they don't have to be big to be appealing, though I do think a lot of guys feel that the bigger they are, the better—including fake ones. I personally think size is overrated; I judge breasts not on size but on firmness: the more stuffed, the more I like them, and if they can stay stuffed and be larger, that's just amazing (and rare).
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And to be perfectly honest, I'm really confused by the appeal of fake breasts. I've touched some, and they do not feel normal. In fact, they feel pretty much like what they are: often overlarge bags of goo just below the skin. Ever sleep with a plastic cover over your pillow (my grandmother used to put these on her husband's pillows to protect them from his oily skin)? Even under the pillowcase, it just doesn't feel right. Same with fake boobs. 7 Body Parts Men Love—Just The Way They Are
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And they look odd, too, don't they? They're always too high and too perky, even if they're not of biologically impossible size (though they often are). So unless you're in real need, I'd advise against fake ones—especially if you're considering a pair larger than a C-cup, at least if you ever want men to look you in the eye again. You don't need them—there are plenty of guys who will love your itty-bitties, who won't mind if one is bigger than the other (which is pretty normal) and will still find them sexy even after three kids.