The park is huge, probably 5 or 8 square miles. It runs from basically the Watergate on the Potomac near the Lincoln Memorial in a northeasterly direction bisecting the District of Columbia all the way across the Maryland line. The National Zoo borders it. It is really a deep (50 to 75 to 100 feet) draw or holler or whatever you call them back east that is several hundred yards wide.
AgentEcho's map at post 42 gives you a good idea of it. Plenty of room for dastardly deeds (done dirt cheap -- for all you AC/DC fans out there).