Originally posted by D*Alex
It's a proprietary BB, CODA brand, unique to Cannondales. You have no choice.
That's only on the Caad6. Caad5 uses standard shimano BB's. The size would depend on whether it's a triple or double crankset. It's probably best to take it to the shop if you don't have the tools to pull the BB out. If you can pull it out yourself the casing will have the measurements on it.