It's hard to get a feel for it, but overhead it's almost as large as the main station:
and hard to tell from here, but it is taller
Overall, it's pretty large.
That said I'd say it's about as useful as a parking garage at Penn Station in New York.
I can definitely see a smaller parking garage working.
There's decent integration with DC Metro (Red Line) inside the station, and the main level of the parking garage is turned over to busses.
And the second link shows the bike parking station and bike lanes.
If you're going city center to city center, it's not overly useful. If you want a lot of parking, go north to New Carrolton or even BWI. Or go south if you want.
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