Proportion

I advocate being proportionate.  Now when you say you're a "C" this is somewhat arbitrary as all bras are different.  However, if you are athletic it is not unreasonable to get, what I call, a sporty reduction.  This means a reduction of 150-400cc that will help maintain a nice figure but at the same time allow you to feel more comfortable while exercising and reduce the strain on your neck and back. 

Unfortunately, there's no insurance company that will cover this reduction.  See several surgeons as well.  Many of us that have been trained later due a superomedial reduction instead of the old inferior pedicle.  It's smarter and safer.