The answer is - the same size that is supported by Windows XP Pro.
I don't recall any limits on hard drive sizes in XP. There may however some limits dictated by the file system used and hard drive
sizes supported by the BIOS on your MB. For instance, FAT32 will support up to 2 TB partitions. 36bit or 48bit LBA support in BIOS
will limit the hard drive sizes as well.
