Maximum drive size for mirrored RAID
I'm want to set up a mirrored RAID with two 4TB drives that I have. When I put the drives in the tower (Mac Pro, OS 10.8.5) and open Disk Utility, the RAID button at the top does not show up for these drives. I only see First Aid, Erase, and Restore. Is there a max size for a drive that I'm exceeding? Or is it something else?