It's hard to estimate a cup size based on an implant volume without you measurements or photo. I recommend a visit with your surgeon that will show you samples of implants that you can try with a bra.
It seems unlikely that a breast augmentation of a moderate size will cause further problems with your back. A heavy or large breast can cause upper back strain when a support bra is required, though lower back issues and herniated discs are not commonly caused by even the heaviest breast.
I generally don't recommend ice for breast patients as the implant is so far from the skin surface it does little good nor is it helpful after the point of maximum swelling, after the first 3 days.Discuss this with your surgeon and follow his directions!#iceaftersurgery#breastaugmentation#BA