The short answer is : 20 GB (give or take). This is also the wrong answer, because all you would have there would be a long, long, unformatted text file – no images, hyperlinks, or even section headers. Great for machine-learning (if that), but little else.
If you wanted a printed, human-friendly version, Encyclopedia Britannica-style (each volume 25cm high, 5cm thick. 500 leaves, double rows of text), one would fill a room with a little under 3,100 books. And it still would be without images.
Or, you could want a human-readable, portable version of the encyclopedia, with images. For this your best (and only) bet is probably Kiwix.
As of October 2022, the Full English Wikipedia (ca. 6.5 million articles), with images will use up 91GB of storage space (German and French, the second-largest: 36 GB). A 128GB microSD card to store it on will cost you less than USD 20.
If you can do without the images (what we call the nopic version), then you are down to 46 GB.
Interestingly, research has shown that 60% of Wikipedia readers could find the info they needed in the lead section and infobox. If that’s all you need (our mini) version, then 12 GB is all you need.
Either way, you can download them all here.