From http://en.wikipedia.org/wiki/Intermodal_container#Types, we can see that the standard "20-foot" container which I assume is what you're thinking of is (internally, since we're comparing with other housing) 18.7 by 7.7 foot, so the total area is just 144 square feet.

Note: computing the above was horrible, how are you supposed to enter 18′ 8 13⁄16″ * 7′ 8 19⁄32″ (the dimensions as given in Wikipedia's table) into a calculator?