Given ridiculously perfect conditions, I don't think there's any (reasonable) upper limit on how big a pyrite cube could grow.

There are certainly limits on how big one could grow in real life conditions, but those are always fluctuating, and who knows when conditions might randomly become just right for an even bigger cube...

At any rate, you'd eventually be limited by your supply of iron and sulfur, I suppose, if nothing else.

Yuge cubes