The ATS has to support the ampacity of both feeds. If the OCP on the utility side is 800A, you need to use an 800A ATS even if the generator can only put out 200A.
With a house that large, I'm betting NEC requires an 800A service, or close to it, so you can't get away with putting a 200A breaker on the input of the ATS, because you'd then fail to meet the service requirements. If you can compute the service size smaller, though, you *may* be able to get away with putting a 400A or 600A service disconnect switch upstream and using a smaller ATS.
