Our family of 5 (kids age 13, 9, and 9) did great in Lara's and Alex's flat for the week. It is a perfect location for touring DC -- you can see the Capitol building just down the road! -- and it is very nice inside and out.
It would be perfect for 4 people (the two bedrooms). It's ok for a 5th to be a child, who can sleep on the pull-out couch cushions.
Most of the time we got around using the DC bikeshare program; there is a station a few minute walk away from the flat. Avoided the cost of renting a car. (Lyft was $78 each way to Dulles.)
With lockboxes on the outside, it was easy to get the keys and arrive whenever we needed. Their assistant, Kerstin, was very helpful. They were flexible in both our arrival and departure times, giving us the chance to get a little extra touring in on the last day.
Would definitely recommend staying here!