The size of the service entrance wire you'll need for 200 AMP electrical service will depend on whether you're using copper service entrance wire or aluminum service entrance wire.
If you're using copper service entrance wire then the size of the copper service entrance wire for 200 AMP electrical service will be #2/0 AWG.
If you're using Aluminum Service entrance wire you need #4/0 AWG Aluminum Service entrance wire for 200 AMP electrical service.