sorry, it's a p5b-deluxe, not a p5b-vm.

whch means it's a:
 ATX Form Factor
12 inch x 9.6 inch ( 30.5 cm x 24.5 cm )

that was a potentially awful error. glad i caught that.

https://www.asus.com/Motherboards/P5B_Deluxe/specifications/