I am scheduled for my BA in 2 weeks. PS recommends 275cc vs 240cc Natrelle round smooth under, advice?

The answer to this can only come from a discussion of your expectations with your board-certified plastic surgeon. There are many things to be considered, and these measurements are just one factor. If you are still unsure about things, should certainly again discuss this with your plastic surgeon, so you know you have the same idea of the expected outcome.