You might try googleing "small cabin plans". Some of these are farily bare and may give you ideas of what the minimum space is. I recently built a small shed from pre-made frames bought from home-depot or lowes, i forget. The frames and wood in general was pretty cheap and one frame was even torn apart and corrected to be even close to the others. Just a warning to shop carefully and look for quality. This cabin probably won't be maintained often and you'll be pretty upset of it's damaged when you show up some time.