I have a custom built rig as well, but opted for a more compact (Corsair Carbide 300R) case. If you have to put it on your desk (and not on the floor or on the dresser) it could potentially take up a lot of space, unless you lay it on its side.
If you put it on your dresser, you should have more than enough space. WashU dorms are big. I know a few people who have custom built desktops with Macbooks for portable use. I personally have a 13" Air, but a netbook could also suffice. I just think the screens are too small lol.
Plus, the screen real estate is nice. Freshman year I made do with an 15" HP laptop screen for an entire year... and hated it. Now I have a 25" monitor that barely fits on the desks in Sofoho. That should be your upper limit for monitor size, by the way... 25". Anything larger and you'll have trouble fitting it if you get the newer desk type that WashU has (the ones with that weird shelf built directly onto the desk).