I would not go under 30' wide. If you do the cost analysis as Eddie suggested I would bet it will take you over 30' wide. 30' means three vehicles or things parked side by side.
Lots of great ideas here but for poles and a DIYer I would use 3 2x6s glued and screwed together - treated ones in the ground and non-treated above the ground. You would have less twist and warpage, you can set the lower part in the ground without having to handle such large pieces, and you can set the rafter in a notch created by the shorter middle piece (easy to lift one end at a time for a DIYer) and individually shim to get everything at the same height. The biggest thing for me is that with 2x6s you get treatment all the way through the wood but this is questionable with 6x6 posts. I understand Eddie's comments about 4' spacing of rafters but there are a lot of pole barns with 8' spacing that have been standing for a lot of years.
I would also go metallic roof for simplicity. Put it up and do not put any holes through it and you should never have to be up on it again. I understand the concept of wanting wood sides for the appearance you are going for.