About... 18 years ago (sheesh!) I built one for my cabin up at the lake, out in the boonies, about fifty miles north of Anchorage, Alaska. We have a fair number of guests and usually go up there for weejends and a couple of week-long vacations in the summer.
When I made it I decided I wanted it to be the last one I ever needed to build.. so I built it so I had 4x4's vertical for underground with 2x6's running crosswise between them to serve as pit "walls". I left about a 1/2" gap between them to permit fluid escapement. The base was sized so sheets of plywood (for the above-ground walls) went on full-size. Above-ground was framed with 2x4's. At that size I had plenty of room to build shelves between the vertical studs for storage on 3 sides (canned goods, bug spray, oils, flammables such as lantern fuel, as well as toilet paper, etc. Also, iy was built from pressure-treated lumber, with the below-ground portion with the additional "ground-contact-rated" treatment. With the brown color from the treatment we didn't bother with paint and even now it looks like new.
We installed a 4" PVC vent from the seat-level through the roof and I located some scrap metal roofing material to put on the top.
The large footprint was also convenient for storage when we weren't up there, especially for 5-gallon cans of fuel, propane, etc. We later built a separate storage shed but still used it for storing a can or two of gasoline, to keep the fumes from permeating everything else in storage.
It wasn't cheap but with a 4'x4'x7' deep hole, I know I'll never need to build another and it won't cave in... ..or as a friend once told me "With *that* outhouse, you can invite ALL of your sh*tty friends up here!"