on the PC it's two discs (DVDs), i bought it 4 days ago and have only installed TF2 so far. i bought it on a friend's recommendation, we always play BF2 and counterstrike source together so i just had to get TF2 to turn him into a human BBQ.
sleek: i assume you mean on the consoles, because on the PC counterstrike source was in the box with the original release of HL2 back in 2004. if you have a reasonably modern PC you should check it out, there are tons of people playing it and lots of mods. the modders have started on TF2 as well; so far they increased the player-limit on each map from 24 to 32, and there are several custom maps as well.
the one bad thing is that PC and console people cannot play against each other, but given that consoles are limited to 16-players (MS must have a reason for that, but i don't know what it is) and that console FPS games typically come with an aut-target feature to compenaste for the gamepad controls it may be a bit difficult to make balanced PC vs console matches.