Probably 2TB for drives. It wouldn't surprise me if 2TB total, given its age, or if >2TB total would "work" but cause issues. I would not attempt >2TB with hardware that old, except in a pure software RAID. The worst possible scenario, which I have personally seen happen, is that it may appear to work fine, until the OS tries to address over 2TB, and then everything becomes FUBAR, because that rolled over and overwrote data starting at byte 0.
Up to 2TB should be safe, having been an MBR and SCSI limit for ages, now.