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What is the maximum size conductor that can terminated on a 200 A 120/240 volt combination pole mounted meter base. Conductor size increased to allow for voltage drop.

By "combination" I think you mean a meter breaker unit.  The incoming lug varies depending on the unit, but for the most part it will either be a 350 AWG or 250 AWG.  The wiring diagrams show the incoming lug size. Online Catalog   If larger lug capacity is needed, we would suggest using something like a power distribution block external to the meter breaker unit to land the desired wire size and then come from the block into the meter breaker with wires within the factory lug range.