2GB has been my rule of thumb, owing basically to the maximum size of drive recognized under early versions of System 7. But this has evidently been a mistake, as that was a theoretical rather than practical limit, and the practical limit is actually a good deal lower. I suppose on reflection that even into the System 7.6 era, drives were mostly under or around 1GB in size (thus 1GB drives or thereabouts shipped even in most of the pre-G3 beige PCI Macs), so this makes sense.