Download Size?

[Arken Guardian]

How big is the game download size?

[TTRM]

If you scroll down to the "system requirements"-section of the store page...
https://store.steampowered.com/app/367520/Hollow_Knight/
...you'll find that the minimum (and maximum) storage-space required is 9GB
That, however, does not mean the game will occupy that amount of space. On my drive, it occupies just below 7GB.

[Apoc]

The depot on Steamdb is almost 7GB and that should be the download size. Installation size might be bigger depending on the game's compression.

https://steamdb.info/app/367520/depots/