It's not hard to figure out dimensions. Who's gonna know they're not exactly the same? Get some scrap carboard, cut it roughly to size and tape it together, then offer it up to your cab to see if it looks out of place.
I regularly have to work out dimensions for custom control panels for customers and the one thing I've realised is that it is the controls themselves that will really determine the control panel area. Plan your surface first, then see if it is confortable, add a slight overhang for 'wrist rests' then just keep the box proportionate by standing back and thinking 'is it ugly?' or 'is it safe?'. Once you've done this you're good to go ahead and cut some wood.