the N5200 is using the ext3 filesystem for the raid. The maximum for ext3 is between 2 and 32 TB, depending on the block size. The N5200 starting with firmware 1.00.06.5 seems to use 4K blocksize which means that the filesystems can be up to 16 TB and the max. file size for a single file is 2 TB.
NFS V3, which is supported by the N5200, supports files with sizes up to 8 Exabyte (= 2^63 bytes).
But you should check if the 750GB hard disks are really supported. And I would contact Thecus and ask them for confirmation.