Size?

Meter socket, sometimes called trim?

Physical or electrical?

Electrically it needs to be rated at least 250 amps. I think the next largest size is either 320 or 400 amp.

You really should check with your utility, as not every utility approves of all meter sockets.

120/208 volt will be a 7 terminal or jaws.

You can start here:

http://www.milbankmfg.com/

We have a 200 amp service supplying a loadcenter rated for 200 amps max. If we wanted to upgrade to 250 amp main disconnect we would then have to get a larger meter (electrically) and meter socket. Then upgrade the loadcenter as well because the panel is only rated for 200 amps? Correct?

First off, you should keep all related info in the same thread. Do not start a new thread to add info or answer a question.
https://www.askmehelpdesk.com/electrical-engineering/what-size-meter-required-250-amp-main-disconnect-208y-120v-3-phase-515111.html


OK, you do not just "upgrade to 250 amp main disconnect". What you are proposing is a service upgrade.
The meter itself is not your concern, it is that of the power company. You do have to get the correct meter can though. The power company will have the specs of what is required for them.

Since this is a commercial setting why not just let your electrician handle all the technical and code issues?