I was in DC for work a couple weeks ago and I went to Potomac Mills and Leesburg. Neither are really close by public transport. You would probably pretty much need to rent a car for the day to get to Leesburg, which is what I did. The day I went to Potomac Mills I took a cab from the furthest out Metro station in that area which is Franconia/Springfield ( I think).
There are lots of ladies here that shop those stores regularly that will probably be more help.
Leesburg is awesome, but definately start calling ahead if you have stuff in mind.