MSS Cubby box is 250mm wide, 420mm box length + 80mm for the CB compartment + 90mm for the bottle tray, 250mm high (to top of box) 330mm to top of arm pad. Still on 40mm blocks.
You could not go any longer than MSS box, but i would go wider, height, I prefer the MSS @ 330mm high.
Be carefull that the box is far enough forward that the arm rest stays open on it's own, my first MSS installation didn't and it really annoyed me, so had to alter..
What you got planned???????????????????????
Andy