How big of a HDD can a Mid 2010 Mac Pro handle?
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]