I've given this a lot of thought over the years. There is never enough room!
10,000 square feet, say 50 x 200 is the place to start. Of, course, you'd still need a fenced acre (maybe 10 acres) to put all the future projects.
My shop is just under 2000 square feet. The main shop area is 30 x 50, which works pretty well. I'm at a point where I have either got to quit building stuff or get rid of tools (shudder).
No matter what size you build, try to keep storage in some other area. Lawn mowers, camping equipment, boxes of kitchen utensils, etc. should all go somewhere else. Consider adding a seperate storage room.
A covered patio is also a nice feature. Grinding, stick welding, and plasma cutting are extremely dirty operations. Doing them outside will keep the shop a lot cleaner.
I can never figure out a good way to store car parts when I'm in the middle of a project. When you take a car apart, The volumn increases at least tenfold. Some sort of organized tempory storage racks would make life easier.
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