Microdots managed about 32MiB / square centimeter (from the "you could fit the Bible 50 times in a square inch") measure. That's a completely arbitrary density to achieve, since it was photographs which were enlarged and shrunk, and you could hypothetically use any encoding for your gold etchings, and it also leaves out the question of "how fine can you etch gold plates while still having the engraving be 'robust'".

But in any case, that gives you a target area of ~325 square meters for a 100TiB archive.

That's a lot, but not like a crazy obviously impossible number like a million square km or something.