Elric of Melnibone is the biggest I’m aware of, it invokes higher infinities in its lore and even a single universe is composed of countless higher dimensions, and you explicitly have to “scale up” to reach that kinda stuff and “scale down to infinity” to move to a lower universe. Discworld is another obscenely big one, a lot of usage of higher infinity related math from what I’ve seen. Transformers is also ridiculously big but how big exactly is kinda hard to determine. It’s got that marvel esque bullshit where each universe essentially spawns an infinite amount of multiverses and there are other multiverses inside of it and the weird Kabbalah shit but yeah, hard to chart it all exactly. I have no real clue now how it compares to other beyond multiversal series because the way its cosmology is setup is just kinda weird but not on the surface. And I know nobody wants to acknowledge it but SCP is probably a contender for the biggest out there now Besides Suggs