It kind of depends on what you're doing. Two of my friends have done the 'double decker garage' idea, which is to put the machine/wood shop on the ground floor and the office/electronics on the upper floor. My wife still believes that cars go in garages so I have yet to claim the full downstairs :-).
Things I find most useful in my space are the electronics bench, the computer "station" (3 monitors on a desk), and the "small fab" bench (3D printer, mini-mill (Sherline), various holding jigs). I've got parts and such upstairs but they could reasonably be anywhere.
My activities are primarily small circuit design/build/test,
small mechanisms (robot) design/build/test, and algorithm research. These things don't have a lot of consumables per hour of engagement so they can go for a long time on relatively low additional investment.
For larger fabrication having a garage door and a concrete floor are essential. But the kinds of things you want to work on and create is pretty intensely personal usually. A friend who likes to build airplanes built an assembly hanger that was roughly 40' x 60'. I have never found the space I have above the two car garage (and the same size square feet wise) to be an issue.
What I have done over the years is look at the projects on the queue, and the things I want to learn, and then built to support those efforts.