Hi I haven't taken the tape measure to it, but it's a little under 4ft deep and 6ft wide. However, because of the roof overhang, that makes the roof space around 6ft by 6ft. It's probably about 5.5 ft high, or a little more.
As for dismantling, I really don't know how easy, or difficult it will be. The sides, back and front are attached to the base with nails (not screws). can't remember exactly how many, but 3 or 4 in each. The roof was nailed on also. These were the fixing that came with it when I bought it.