Depends on the girl, really. If she's supposed to be athletic or specialize in combat and shes not an android or full-body cyborg, then it doesn't make sense for her to have absurdly large tits since all that physical activity will burn away the fat in her chest as it does everywhere else. It's why most IRL female athletes tend to be cutting boards. That said, the limit for those types would be somewhere around Saeko's size (see picrel).

However, if she lives a relatively sedentary lifestyle, such as that of a lazy NEET/hikkikomori, then it would be much more believable for her to have ridiculously-huge honkers.