Okay, first and foremost, remember the Golden Rule: There is no such thing as a shop that is "too big". (At least from a space standpoint- affording it is another matter entirely. ) Like a gas, the stuff that goes in a shop expands to fill all available space.
I know several people who started out in a garage, outgrew that, built a shop, outgrew that, built a bigger shop, and have now outgrown that, and are looking for something even bigger.
Even a 100x100 building can get filled up fast, if you're a packrat, have a bit of disposable income, and a wide circle of friends.
Second, it very much depends on what you're planning to do. Obviously the guy that plans to spend his retirement building miniature model steam engines doesn't need as much space as the guy who's going to do a full rebuild on an old Cat 966 endloader.
And third, you're going to have to ask yourself just how firm you're going to be with yourself about collecting tools, parts and junk. Will you install a basic set of tools (hand, machine, woodworking, what have you) and then stick with just that? Or do you expect to keep swapping and upgrading and adding to the collection?
Personally, I have something like 1400 square feet of shop, not counting storage areas. Part of that is "machine room", the rest is car bays- one of which is currently filled with a very large lathe.
To do maybe 85% of my paintball work, I could, if necessary, pare that down to maybe as little as 250 to 300 square feet. It's not volume intensive, and I do the majority of my work on just two of the machines.
However, for working on a car, especially if you're doing something like a body-off, even a full 1400 isn't quite enough, since some of it is of course filled with tables, workbenches, tools and storage. You start taking sheetmetal off, and it's either outside in the rain (unless you live in Southern California) or it's stacked inside somewhere.
Personally, if I had the choice to do it all over again- but NOT assuming I had any kind of unlimited magic budget- I'd shoot for something around 2,000 square feet. One single room, two car bays, a select set of machines and workbenches along one wall, movable work tables and rolling tool cabinets along another wall.
If I had the budget and space for it, I'd probably bump that up to 3K/ft. Keep the central 2K box, and have two additional rooms of about 500 square feet each. Either along the side walls (which I wouldn't recommend) or side by side on an end, opposite the car-bay garage doors.
These two would be the "clean" and "dirty" rooms. The clean room for the machine tools, so you don't have to worry about getting grit and dust on them. The "dirty" room for the stuff that makes dust and grit- the sandblaster, bench grinders, arc welders, cutting torches, plasma cutters, chop saws, etc.
Tuck a small bathroom at one corner and you're set.
If you want to make the place integrated, have it two story, with the house/apartment above the central section.