Depends on what space you have but i'd say the biggest capacity 11.1v you can fit in there, you'll need it.
Yes standard rifle capacity batteries will work but you'll need to change them regular if you're putting out a lot of suppresive fire (and if you arent putting out a lot of suppresive fire why the hell would you carry an m249 )
unless its been up graded it's not designed to run 11.1v capacity is more relevant the voltage (though lipo over Ni-Mhs every time) i could run 6000 rounds through mine on 7.4v 5200mah lipo no problem.