I was playing around in #apple2 #pascal in the Virtual2 emulator this morning, and since the emulator supports a virtual hard disk (for Apple IIc models), I experimented a bit and determined that the largest volume size that Pascal supports is 32767 512k blocks, or about 16MB.
I'm not really sure what you could realistically do with a 16MB disk, with a non-hierarchical filesystem that supports a maximum of 77 files. But it's an interesting data point.
I know that other UCSD Pascal systems support splitting a single hard drive into multiple filesystems, but I'm not sure at what level that is implemented.