So... I'm thinking about using gogrepo.py to backup my entire GOG library. But it'd be great to know exactly how much storage space it's going to take beforehand. Does anyone know of a way to calculate the size of the data without going through it all manually and adding it up myself?

Hmm go grepo has a -dryrun option so I suppose you could use that and pipe the output to a text file.
Not sure if it does a total though.

While it probably won't help you with each individual game, I can tell you exactly how much space the 1454 games I own from the GOG catalog take up.

Notes:

- I have only downloaded the Windows version (not Mac or Linux) for each game;
- I have downloaded all of the 'extra' content and goodies that were offered for games.

The total size is 1.82 TB, which I have stored on an external 3 TB HDD.

Indeed it did! But apparently it doesn't have a way to calculate the total sum including Windows + Linux files plus extras.
It does give me a rough idea though.

Yeah, it's useful.

Since it's been solved, I just have to know the answer to this question: what's holding you back from getting the final 23 titles? XD

And somehow I thought more space would be required but I guess a whole mess of titles are under 100MB, balancing out the 10-30GB monsters

It took me a while to figure out how to actually force MaGog into only showing my library. Turns out I only need 78gigs! (At least it sounds like I'm downloading Windows installers only though, no file sizes for the bonuses).

Funny you should ask. :)

Actually, I think there are probably much less than 23; some of them games I just didn't outright purchase because I didn't feel they were worth getting, however, several items in the catalog were repetitious due to them being the +1 or +2 edition of the games offering bonus content like soundtracks and whatnot. So, for maybe five or so of the games, they had two other editions with bonus content I chose not to purchase...

Some of the games that I have are also ones that have been removed from the catalog for some reason or another, a few of them had been removed, but later re-added.

I also tried to stay away from the real "stinkers", mainly those which received a collective review under 1 1/2 stars.

I doubt it's 23; it's probably a different number. Different versions/DLCs and the like are counted separately in the catalog and the library. For instance, Risen 2 has 3 entries in the catalog (Base, DLC, Gold), but only 1 in the library.

On the other hand, unbundled games can take more than 1 entry in the library, but only 1 in the catalog. For instance, Leisure Suit Larry Greatest Hits and Misses takes up just 1 entry in the catalog, but a whooping 8 in the library.

Why didn't you mark Smannesman's answer as the solution? Since you are going to run gogrepo anyway, it will tell you exactly how much space your whole download will take right in the beginning when you start the actual download (ie. how many gigabytes are still left for download before you're done). It will tell this for all files you are going to download, e.g. Windows + Linux + Mac with various different languages and all extras, whatever you decided while running the update command (by default the update command selects only English Windows versions with all extras, but those defaults can be overridden with options).

I guess you can either use that -dryrun option with the download command, or even if you run without it, simply cancel the script after it has started downloading.

He didn't exactly know if it would give me an output of the space. I am still waiting for gogrepo.py to update, over a couple of hours now and it's not even halfway done, so I can't run the --dryrun yet.

Yeah running the update command can take a damn sweet time when you have lots of games.

It appears so. :)

But it's worth the wait! :)