I'm assuming this is just a place for the hens to sleep and lay eggs in - in that case I'd imagine a 5x4 footprint is enough room for a dozen hens to roost for the night; chickens pack in like sardines when sleeping and don't hang out in the actual coop unless you don't have an area to protect them from the elements in bad weather.
The simplest way to extend the existing coop would be to do what was drawn by pibb. The existing 2x4 protrusion should be fine to attach from assuming its all structurally sound; I'd keep all additions a lightweight as possible as this is a mobile tractor and you're going to be changing the weight distribution/balance point of the existing wheels. My rabbit hutch tractor is smaller than that and not exactly light when moving it around - I'd imagine yours would be quite a chore to move once you add onto it, if not moving the wheels forward.