The main thing that will affect fish size is the population dynamic between the two species. If you let the bluegill overcrowd, they will be much smaller on average, but your bass will be larger. If you let the bass overcrowd, your bluegill will get huge, averaging 9" or better if you fertilize, and especially if you feed; but your bass will be significantly smaller. If you keep both species from overcrowding through diligent, well-tracked harvest, you can expect to catch good-sized individuals of both species.
I would definitely stock CNBG, as there's no downside to them where your pond is located, and they'll get significantly larger than normal bluegill.
