definately 2x6 walls for the reasons described.
might also want to include a 220 power supply in case you have need for a welder, stove, that kinda thing. I also prefer two doors as apposed to one single one.
might want to also consider a 8ft door, its easier to go bigger than to have smaller and have a need for bigger.
in my case, I wish I had made my garage bigger and with two doors or possibly 3
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