How big can I go for a subpanel?
Since your main breakers are at the meter, each "main" panel can be as large as you please, but of course each must be at least 125A.  The 125A breaker protects the wire to each panel, and the panel itself. 

Definitely don't scrimp; nobody on this forum ever complained about having too many spaces in their panel, but we get lots and lots of the other. 

If you have two 2-pole breakers, you will need 2 panels.  Paralleling two feeders to 1 panel is not allowed except with very special equipment.