Depends on the girl.

If she's an adult who lives an athletic lifestyle or specializes in melee or hand-to hand combat or a teenager, then they should be no bigger than her head.

If she's an adult that lives a relatively sedentary lifestyle i.e., a typical NEET or hikkikomori that subsists on junk food, then 2 or 3 times the size of her head should be the limit as anything bigger would be grotesque.