I am HAVING a meltdown!
I have added A TON of special regions to my Regions.xml and now I forgot what I am doing!
You take X,Y from NW corner and then X2,Y2 from SE corner.
X - X2 = A
Y - Y2 = B
so it is
X,Y,A,B
NW corner X length, Y height Region bounds use NW coordinate then X # of tiles, Y # of tiles *see picture for regions that aren't equally square you must repeat starting at each NW corner.
I still use Dougan Ironfist's Region Editor. It does work with ServUO and it's fairly simple to use. I have used other region creation tools as well, like Regions-In-A-Box (which has a flaw in that if a worldsave hits before you set the region boundary, the shard crashes), but I prefer the Region Editor personally.