I think the street fighter concept called for 12 man crews and ships that were about 500 tons. I think that would be a good size for a series of primarily autonomous ships. The thing about a "robot" ship is, you would still require a crew on shore somewhere.
Or you can have the crew on a ship nearby. Something like a bigger LCS, equipped with a comprehensive AAW suite, or something of a hybrid of a DDG and an LPD with a well deck so it can act as mothership. It could operate far enough offshore such that the enemy would have to deploy its air force to counter it, where the AAW suite or carrier cover would then come into play. Amongst other things, the unmanned vessels could even eat missiles to protect the actual crewed vessels, and the crewed vessels can destroy the unmanned vessels either through armaments or command directives if a hacking threat is imminent.