What is the largest hard drive a Mac Pro 3,1 (early 2008) will support?
I don't know any upper limit, but after checking more carefully, in mine I have 


1 x 3TB
3 x 2TB
1 x 1TB [SSD in the spare optical bay, wired down into the 'secret' extra SATA slots]


Late addition
It appears that there is not necessarily a data limit but a physical limit.
New 6TB+ drives have the screw-holes in a different place &amp; will not physically fit the HD sleds in a 3,1.
So it turns out that the limit is 4TB - the largest with the old screw-placement.

You can get replacement sleds for a 4,1 or 5,1 from OWC, but not for the 3,1