Like Half-Life 2, Team Fortress 2 and other games on Steam, the Disc will be outdated and you'll basically be downloading the entire game through Steam anyways..... I've tried to install both HL2 and TF2 by using the discs I already have, however the discs prevented me from installing the game unless I was hooked to the internet and had Steam running.
I haven't counted up the total size of the patches or how much you'd have to download all at once, since I've been getting the various patches as they become available..... but my guess is the patch would probably be more or less the size of the entire game itself.
One of the larger patches a few weeks ago focused on some performance boosts and modifications to the maps, and that patch was quite large because we all had to re-download the maps again for the performance boost to take effect..... so expect a large update/download.