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It says on the store page its 5.9 GB but on steams ad it says it is 15 GB.
The reason i ask is my DL speed isnt that great
The difference may be download size vs installed size.

With the GOG downloader enabled, the Windows installer is listed as 5.9GB (all the bonuses combined are 1.9GB, though 1.1GB of that is a video), while with it off the installer is in 4 pieces of 2GB each and 1 of 32MB.
I got the GOG version as a digital backup, but have the retail version of the anthology, so have not downloaded it.