As for the absolutely smallest to house your tractor (and I contend that you'd be foolish to make it this small) is 2' on each side for access, 2' at the front and 4' at the rear. If you have an implement attached when it's put in the shed, I'd add the length of the implement plus 2' more. So width of tractor (or widest implement) + 4', and length of tractor + 6', or length of tractor and implement + 8'.
Note: This does not give you much room, about all you'll be able to do is check oil, etc and get on & off.