I think, somewhere between "half the size of her head" and "twice the size of her head" there's a reaction of "okay, that's just ridiculous". If massively-endowed women show some realistic effects like some sag(age-appropriate, though even teenage girls have boob sag if they're big enough) and realistic shape(perfectly symmetrical beach balls aren't realistic, unless they're implants), then a bit bigger is fine. Big boobs that don't call attention to themselves in the wrong way, basically.