I heard somewhere that the maximum HDD size that my Mid 2010 Mac Pro can handle is 4TB. Is this correct? If not, how big can I go?
It's not that they can't address a larger drive, it's that the screw holes don't fit the 'sleds' that the drives are mounted on.
OWC sell replacements - http://eshop.macsales.com/item/OWC/MPRODBKTLG6/ though I have no idea if they ship outside the US, or indeed if that would be worth it.
Standard sled - 4 mount points.
6TB compatible sled - 6 mount points
Pics from OWC [no affiliation]
The OS X HFS+ Operating System will max out around 8 EB (Exabytes)
To put that into perspective:
8EB = 8,000 Petabytes
1 Petabyte = 1,000 Terrabytes
1 TB = 1,000 GB
In other words...
8 EB = 8 Million TB or 2 million of your 4 TB Hard Drives. I hope you have room under your desk.
However, a 4 TB drive will easily be installed or you could opt for one of the new 8 TB Helium Drives.