Bigger = Better. No realistic upper limit.
BUT there is a caveat:
Hips / Butt / Thighs should be wider than the Boobs. If the boobs are bigger than the lower body (very common in manga / anime) it just looks wrong. They need to be counterbalanced.
And frankly, bigger all over is better, as long as it remains proportional.
Also Glasses. Can't forget the glasses. Fat girls with glasses are the best.