It is as simple as copying the folder and pasting it onto your USB, then when you're ready, pasting it back into the Electronic Arts folder.
I copied the entire Electronics Art folder, just to make sure everything went okay.
As for space, I'm honestly not sure how much it takes up. I put it on an external hard drive which had 500Gb of space, so I wasn't exactly working with something with limited room in respect to the game. On the back of the disk cover, it says to allow 1GB for the base game, and 1GB per expansion. But to be completely honest, I'm not sure whether that affects how it will hold on your USB. You may need to wait for someone else to answer that part.