We've got this that lists the size of Nintendo libraries up to the Wii: https://www.reddit.com/r/emulation/comments/3h2fqx/how_much_space_it_would_take_to_store_every/
Best way to do it is to either have the entire library for older systems since newer systems take up more space, or go through all of them and count their exact size. Not pleasant, but it is what it is.
"NintenDON'T sue me please haha"
That would be interesting. You can find the raw size from redump/no-intro dats, but I don't know of a non-scraping way find the total compressed size of a set.
The PS3 fullset is roughly 33 TiB raw. If the downloads are encrypted then that probably translates to 32 TiB compressed as most of the sectors will be encrypted and so incompressible. If the downloads are decrypted then that probably translates to around 20 TiB compressed for the full set.