Go to this jsFiddle, bookmark the link (it's a bookmarklet), go log in to Steam on your browser, look at Community -> Games -> All Games, use the bookmarklet (open it like you would a bookmark). You'll get an alert telling you the size on disk and the size you'd need to buy, approximately.
That being said, here's how you use it:
1. Go to this jsFiddle, right click the link, click "Bookmark this link" or whatever equivalent your browser has.
2. Go to Steam, make sure you're logged in to your account, then go to Community -> Games -> All Games. Make sure you're looking at your profile, unfortunately you can't use this to calculate the total size of any of your friends' libraries. Once you're at the All Games page, click the bookmarklet in your bookmarks, and it'll do its thing.
3. You'll get a popup kinda like this. The difference between "real" gigabytes and "marketing" gigabytes is a boring, complicated mess that nobody cares about and thankfully is going away with SSDs - just know that the "marketing" size is the amount of space you'd buy, the "real" size is the size you'll see on your computer if you install everything.
Now, there are a couple of caveats: this isn't using any official Steam feature, so the method may very well stop working at any point without warning. Another problem is that at least one game is messed up on this page; see where it says that Trine 2 takes up 13.7 MB of space? That's filthy lies. The game takes up about 1.5 GB of space.
All that being said, however, this should give you a reasonable idea of how much space you'd need to store your entire Steam library on a disk. Just go up a couple of steps (plus however much more you want to store the rest of the kipple we all carry around everywhere between hard drives), and you should be good.