Though the latter bit about the ‘size of economy’ is doubtful to me: the populations of top MMOs are in millions each, but surely the players don't spend the sums of their monthly rent and food bills in the games. This report from January 2021 puts the daily volume of transactions in WoW at about two million USD: https://massivelyop.com/2021/01/21/world-of-warcraft-players...

I eyeballed the rate for late-ish 2021 from this hastily found site, at 20 bucks per 200k gold—seemed to change little in the past year, so hopefully holds true for January 2021 as well: https://www.playerauctions.com/market-price-tracker/wow/