I think Midway is actually alright with regard political restrictions but just has a short runway. LGA has political restrictions if I remember correctly (as does Reagan National) in both the game and real life. Someone correct me if I'm wrong - I don't really fly to the US with any of my airlines.
At LGA, the range is limited to 1500 NM while DCA is limited to 1250 NM in game and IRL. There isn't any size restrictions that I know of besides the operational restraints of the airfields. As of now, the largest scheduled plane that flies into DCA is a Delta 757-300 but 767's and 787's have visited in the past.