how much total space does all the steam items take up?

The op clearly stated EVERYTHING on steam, you can't exclude anything even rereleases of games. The store has over 12,268 items currently for sale, and i am sure several thousand more that are still on steam that are no longer available to purchase and thus are not shown in the store. There are also many more that may still be available but not available in my region.

Other things you fail to include are things listed under tools which are not included under your owned games list and are not listed on the steam store but are still on steam and are under the umbrella of what the op is asking for. I have 128 tools on my account, around 40 or so are under 100mb, around 30 are between 10 and 15 gigs, around 10 or so that are between 16-20 gigs with the rest between 1 to 9 gigs. Decent chunk of space there and i don't even have all that many games on my account.

Another big one you don't take into account are screenshots on steam, each screen shot appears to be around a .25mb a piece and after a bit of searching the max number of photos you can have on steam cloud is around 8,000 which would be about 1.75gigs (assuming maths are correct) for a single person who capped out their screenshot cap on steam cloud and i am willing to be there are a fair few people who capped it out. There are over 100 million steam accounts, lets assume 50 million of those took at least 8 screenshots, that would be over 100 terabytes of space just in screenshots alone not counting people who uploaded more than 8 screenshots or people who capped out the limit.

Remember, the op is asking for space used by everything on steam, not just games and software currently for sale.

/r/Steam Thread Parent