If you are interested in popular acts that don't play the mega-amphitheaters or the arenas, we have several good venues. I gather that's the thrust of your inquiry, rather than the specific size of the venue.
In DC,
Black Cat and the
9:30 club (under same ownership in same neighborhood) are your best bets. A newer venue is the
Rock and Roll Hotel. It seems cool, but it's more of an indie venue than a place where you'd see national acts. All of these clubs skew mostly toward rock of various genres. At Black Cat and RR Hotel, large crowds will turn out for indie acts I've never heard of. However, this makes for great shows as the audiences are usually pretty fired up.
In northern VA, two great venues are the
State Theater and the
Birchmere. Both have seating with dinner service for most shows. Ticket prices are reasonable too. Acts range from acoustic alt-country duos to a local 80s cover band.
In MD Rams Head gets plenty of "mid-level" acts such as Moby, Black Crowes, and Lucinda Williams (all on the Sept. schedule). As well, Blue Oct. played there this past weekend.
Outside of these rock clubs, Lisner Auditorium on the campus of George Washington University and DAR Constitution Hall downtown also draw in groups that are popular but without the local following to pack an arena.
Though the Kennedy Center and the Warner Theater do pull in some contemporary rock acts every once in a while, that isn't really what the spaces were designed for nor is it often that you see many such performances come to either venue. For example, I think the only popular musician to come to the Kennedy Center this fall will be Ben Folds who will be accompanied by/accompanying the National Symphony Orchestra. Justin Timberlake at the beginning of his decline played the Warner Theater. Accordingly, rock lovers don't pay too much attention to either location. (Both are great for the performing arts in general, though, and I've seen many performances at each venue.)
Merriweather Post Pavilion, Wolf Trap, and Nissan Pavilion are all large outdoor ampitheaters near DC. At these sites you'll be more likely to see the Killers, Depeche Mode, Motley Crue (yes, they toured this summer), and other large acts. It's unlikely that Bonnie Rait or the Decemberists would play shows there unless part of a larger festival.