I think you can get any size, just a matter of the placement of the cable connectors and size of the HD. Some Hds are slightly thicker then others.
You want a 7200 rpm hd, 5200 are slower.
Its a Ultra ATA, so make sure you dont buy a SATA.
Try this link and see what other people have done.
http://www.digitalhome.ca/forum/showthread.php?t=73790