Whatever you build, you will likely wish you went bigger.
Mine is 52' x 24' x 16' sidewalls. I have a single door that is 20' x 14'.
If I were building again, I would go 28' wide, as I could have done that without significant changes to my back yard.
If you want more than one vehicle in there, it is much easier to be side by side than tandem. Whatever is in the back is what you will want out.
Remember to do a lot of power, a lot of light, a lot of outlets (both inside and outside). Windows and sky lights are your friends.
Attic fans or ceiling fans are a good idea as well. Without heat, my building gets very cold in the winter. And even in Denver, it holds the heat in very well in the summer (read gets real hot).