Bob,
That sounds like a very nice size. I've actually come to believe that there is such a thing as too much. For me, when I get too much space, I get too much stuff. Then I spend more time maintaining the stuff than I spend using it. Not my idea of fun.
That said, I'll be a bit envious, since my present garage is a smallish 2-car garage and has to handle woodworking/carpentry, welding, automotive/motorcycle repair, machine tools & my wife's stained glass studio. It gets a bit tight in there. I think somewhere from 1000 to 2000 square feet would be a good range for us.
In my opinion, the one thing missing from nearly all garage/shops is adequate lighting. I got a bunch of free fixtures, so my garage has twelve 4 foot 2-tube fluorescent fixtures plus four of the old green enameled barn lights with high wattage compact fluorescent bulbs. I consider it just barely adequate for general working light. I wouldn't mind at least twice the light. Light make a huge difference for doing quality work. If I ever get to build the kind of shop I really want, I will get really serious about the lighting.
You might want to consider having walls between the metal working area and the woodworking area. I also like the idea of having a concrete floor for metalworking & automotive, and wooden flooring for woodworking. Also, figure out how many electrical outlets you think you might ever need. Then double the number. Then add some more. It still won't be enough.
For working on cars & tractors, a grease pit would be nice. most under-chassis work can be done without jack stands or a creeper (I HATE creepers). An overhead hoist is useful, too.
Who's next?
Gravy