Sun's site shows the largest drive they provided for it is 4.2GB, but I don't see why it couldn't support any SCA SCSI drive.
Heat is probably the main factor you have to consider.
I'm fairly certain we had a 9GB drive running in one, but it's too long ago to remember.
Or are you talking about using an external disk pack?
Again, I don't believe there to be a limit, but it'd be really slow