A good rule of thumb is that for every ~150cc's you go up in an implant, you are going to increase about one cup size. With respect to the weight of the implants and potentially causing back pain - as long as the implants are moderate size, they shouldn't cause any significant back pain due to the added weight. For the sake of example, a 10.75 oz can of Campbells soup weighs 305 grams. That's about the same weight as a 300cc implant. So if you imagine supporting the weight of two soup cans, that would be similar to supporting the 300cc implants, which would increase your chest about two cup sizes. I would also inquire as to the kind of "look" you're going for with your augmentation. Do you want a natural look? An augmented look? To you want more projection? More fullness up top? Whenever you speak with your surgeon, I would recommend finding pictures of someone who has a similar body type to you and an attractive chest. That goes a long way to help your surgeon choose the right size, projection, and placement for your implant.