The biggest I have tried as 20 gigabyte (used a  powermac laptop hard disk with adaptor) .  Only issue is having 10 partitions as that fills up the screen with drives.  The Mac is limited to 2 gig partitions or less.  

I know the ram limit is laughable as I have a 63X series board with 2 ram slots.  one marked 4 and one marked 32 and I put 32 in both and sees all 64 plus the onboard.  Not sure 128 would work, just remember at the time those may have been the ram limits per chip specs at the time.