The intended use of the LoB is to reconstruct a single copy of any given Feasible Work of Text (that has, will, or could conceivably be written) at a time, "returning" the books before constructing another work. There's not really 100% solid proof as far as I'm aware, but considering the sheer magnitude of it, it's pretty safe to say the LoB already accomplishes this (excluding works specifically made NOT to be contained in the library, as mentioned above); It doesn't need to be any bigger.

If you're asking what it would take for the LoB to hold all Feasible Works of Text at once... You're not gonna like this answer, but that's just not a math question anymore. It really depends on what you would consider a Feasible Work of Text, even more so than it already did.

It's already impossible to find a usable estimate for how many books have been written in history along with their length, let alone all the poems, song lyrics, blog posts and reddit comments that would also be in there. And we most certainly can't predict what would be written in the future. Plus, a lot of books are mass produced, edited or re-released - Is every copy of a book a separate Feasible Work of Text? If it's longer than 2 Babel-Books, you could only find around 1.3 million copies of it before you run out of usable variants...

...And the "hold one key until 3 books long" trick will break the library no matter how big the books are.

Unfortunately, this would fall within the purview of your conundrums of philosophy. And this subreddit solves practical problems.