If I want to use Windows Disk Management set up a RAID, what is the largest size it can handle? 2TB, 4TB? Also, if it is limited to (let's say) 2TB, can it handle multiple sets?

As far as I know, the largest volume that XP can handle is 2TB. This is not a limitation of XP per se, but of the MBR disk structure. If you want to use the space beyond the 2TB mark, you'll have to move to another disk type, known as GUID Partition Table, or GPT. You'd need to move up to Windows Server 2003 to get this ability.

Have a deeper read here.

technically XP will not support raid unless you replace some files in it with files from server 2003. I wouldn't exactly call the "striped volume" raid. You would really be better off with a hardware raid.