Depends on the girl, really. If she's supposed to be athletic or proficient in combat, then picrel is the limit. Anything larger, and I can't take her role in the story seriously at all because they'd likely be a liability in combat (she'd have trouble putting her hands together while holding them in front of her to properly grip a gun, rifle or sword and being so physically active means she'd have less body fat anyway). If she lives a relatively sedentary lifestyle, the limit can be much higher, though in that case, I'd draw the line at being so big that they completely cover her midriff.