I should really learn my DC geography better before I speak, huh
I think a drive hit fair over the right-field stands might land in the Navy Yard or might, just maybe, drop into the river where the Yard used to have piers. Probably about a 600 or 700 foot homer. Otherwise, a mammoth foul over the press box, and I haven't noticed a foul ever get that high.
(Note: Frank Howard once hit a ball over the RFK / DC Stadium foul pole, just fair according to the LF but the umpire could judge. The foul pole did not go up through the roof. Only fair ball hit out of that stadium.)