You can use GOGRepo for this. It has a dry run download feature after it generates a manifest that can tell you how much is needed to be downloaded. It does take a little setup and requires a little bit of command line interface in Python. I'd put it at intermediate level as far as technical difficulty.
EDIT: Link to Kalanyr's fork GOGRepoC.
https://github.com/Kalanyr/gogrepoc
Link to GOGRepo Thread:
https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e

Yep, gogrepo.py is possibly the only way.

That said, it isn't perfect ... but certainly a lot better than nothing.

I am guessing no method/app/script would be perfect, unless GOG developed something themselves.
The reason being - Updates.
Some you might want, maybe need, others you wouldn't. Determining that adds a level of complexity.
And many updates are huge.
And many games in your library could be in a state of flux, so could change their status during a lengthy download process period.

But at least you could gauge a maximum space requirement ... at time of query anyway.

Thanks guys.

I would need a 7tb HDD or multiple 2tb NVME/SSD drives to store my whole collection. Waiting for them nand flash prices to drop and see a standard SSD with 7tb go for like $200, Ah that would be the day :)