Fastest answer I could find was from 2021: 285GB.

https://stackoverflow.com/questions/65995150/is-it-possible-to-find-out-the-total-size-of-all-existing-npm-modules-in-the-npm

So make sure you have over half a terrabyte of free storage before you try this. These libraries can do things on install, like download and compile binaries. Then there’s overhead for inodes and such, since we’re talking about millions of files. So the impact to the filesystem is going to be much, much bigger than any figure cited like the above.