What size meter socket
There is no reason why you cannot install a larger meter base than the one required. Keep in mind that equipment is rated to handle maximum currents not minimum. In your case a 100A meter base can only handle current up to 100A. Whereas a 125A handles up to 125A and will handle a 100A current without a problem. 

Her's an idea. When I was working we always installed a 200A meter base since know you have the ability to upgrade a service if needed later, and comparing material cost the upgrade was minimal and labor is more or less the same. The only reason we would install a smaller base would be because of space restrictions.

Good luck