Well to begin with, Raid6 requires minimum of 6 drives. That's what I have for my current setup.
Windows only sees the total size of the usable Raid setup. So, if you have 8 drives in a RAID6, that's 6x320 = 1920MB. Depending on how the system actually calculates all the MB in that ..... it should be under the 2TB limit (I think).