I built one a while ago that turned out OK. It is 30" wide and 40" deep, but these dimensions are the absolute minimum you could get away with. You can barely turn around in there.
I used that clear fibreglass wavy stuff that you put over a porch or so for the roof, with a fair pitch. This way there is lots of light without a window, and it sheds the snow.
It is set up in a very rocky area without much topsoil. We were able to dig down maybe two feet; then I put a 45 gallon drum with a rusty bottom in the hole. (55 gallon I guess for you in the US). I had a load of very fine gravel brought in (local stuff, kind of like river run mixed with sand) and built up the area around the drum. There was enough extra to make a nice pathway over to the outhouse with a gradual rise etc.
This fill packs down very well so I tamped it, put 2x4s of preserved wood around the perimeter of the bottom of the outhouse, and tilted 'er up. Been working great - except for the size as I mentioned. I think even going up to 36" x 48" would make a lot of difference.
Jim