The first one I built was five feet deep, nine feet wide, and five feet high at the back, six at the front.
The next two were only six feet wide. At six feet wide, five feet high, and five feet deep, they hold about a cord of wood.
Having them in widths of three foot increments allows me to cover them with three-by-six foot sheets of steel - readily available and in manageable sizes.
I cover the sides with odd boards and slabs from my slab pile - stuff that is destined to burn anyway. I leave one to two inch gaps between the boards. This is enough to allow good drying, while keeping off the bulk of the rain.
I face these generally towards the south.
It remains to be seen how well they work over the course of the winter, but for now, I'm excited about how easily they can be constructed and how well they have protected my extra wood.