I'd suggest starting with a 60x100', but make sure you situate it on the lot to allow for another building in the future. Why?? Cuz once tractor fever sets in you're gonna need the room. My dad has it, he's at something over a dozen tractors now, I was there over christmas and he had 2 that I hadn't even heard about yet.
Just a portion of the collection:
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Dad's new shop is only 28x32x12, built specifically for this hobby, could easily get 3-4 tractors in it, but he has another 54x60 building for storage. Only tools and the current project goes in the new shop.