I rented a large locker at DCA this past weekend. I think they only have two sizes at DCA the small ($7) and the large ($10). A lunch pail (I'm thinking about my Superman one when I was a little kid) will fit into the small locker.
The lockers seem small when you look at them, but they are surprisingly spacious on the inside. In the large we fit two camera bags, my sister's handbag, and my brother's leftovers from Ariel's Grotto.
It was really great not to have to carry all that stuff around, but still have it available without going back to the hotel. A good deal for $10