I just read that 300 cc's is about the equivalent in size to a small grapefruit. Is this accurate?
The simple thing to do is ask your PS for a 300cc implant to feel and wear. Also most PS have external sizers you can try on to see the rough volume of each implant. Better still go to a PS who has 3D photography which will help you pick an implant that fits your body. If you want to use a domestic appliance to measure, then get either 300mls or water or 300gms of rice and put it into a freezer bag. This should also give you a rough estimate of volume but certainly not shape and size. I hope this helps and good luck.