How much space does a cat need?

http://en.allexperts.com/q/Cats-1606/2008/9/confined-area.htm This guy keeps his leukemia positive cats separated from other cats, with some in rooms as small as 10'X10', and I've had 2 cats before living in my room (12x18), and they had access to the master bathroom through a cat flap I had installed in the door (8x10), but I also spoiled them, (9 ft cat tree/toys/interacted with them a lot) and they seemed really happy.

I did take them outside occasionally on a leash which they liked. From what I saw they were pretty happy, if you're limited on space try to add more vertical space for them to play or hang out on, bookshelves, cat trees, or hammocks/perches attached to the roof (doesn't need to be girlish or look like it's owned by a crazy cat lady, I'm a guy and my room is pretty modern looking), just give them more stuff they could do when you're away. Oh yeah try to keep it clutter free, it gives the impression of more open space/less confined.