What's the total size of all games I have on Steam?

Not to bump an old thread unnecessarily, but I just finished building the tool I think you're looking for:

MySteamGauge.com

Your profile has to be public and it's not perfect, but should give you a pretty close idea of what kind of hard drive space you'll need to install your entire collection (or on multiple drives, per Steam's new feature to specify install location).

Cheers!

[Edit] Just realized a screencap might be nice:

Short answer

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.

Longer answer

It turns out that when you visit the All Games page on your user profile, Steam stores information on them in a Javascript variable embedded in that page named rgGames. After a bit of messing around, I created a bookmarklet that will inspect that variable, sum up all the file sizes, and tell you the total size in both real and marketing terms.

The one caution I would have is that bookmarklets are a pretty insecure thing to use, so I strongly urge that you only do this if either you can understand Javascript well enough to know that my script is not doing anything sneaky, or you trust me. _^muahahaha A bookmarklet can do almost anything you can do on a website as a logged in user, so I could, I don't know, make all your games start installing on your computer or something (ssl should block the script so I don't think anything super important is vulnerable).

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.