The house I grew up in had a 2 car attached garage, we built a 40x60 steel building with lots of space around it for parking vehicles. Wasn't enough.
The house my parents live in now has a 2 car attached, 60x100 steel building and a 10x40 storage container that was built into a barn like structure with lean-to on each side so that added 40x10 each side plus storage above the container. All build very nicely and weather tight. Still not enough.
That being said, currently the 60x100 building has 3-4 RVs, 3-5 boats, 6 ATV's, multiple lawn mowers, all holiday decorations, any of my brother and I's random projects ranging from shooting benches, cars or pieces for his horse barn, typically a vehicle or 2 and a compact tractor with all it's accessories. Plus all the odds and ends stuff that collects. There is a LOT of stuff in there.
As said it all depends on your needs. My old man needs all that space and more because he allows other people to store stuff there and he has a lot of stuff himself.
Another thing to consider is door size. I really wish I would have gone with wider doors on my recently built home. I have an 830 sq ft 3 car garage attached but getting 2 cars through the one large door is tightish and makes my wife nervous, another foot in width would have been a really great idea. Not too big of a deal as I am planning a 40x60, maybe bigger shop in the future.
As has been said before, you never hear anyone complain about too much space. Just like with gun safes, bigger than you need allows for future expansion/collection.
Ideally, something big enough to have a 6ft perimeter around anything you may need to work on, dedicated welding and wood work areas, paint booth, full restroom, lounge/bar area, kitchen, wash bay, and a go cart track. Should about do it.
Oh and a fireman's pole. Always tell your wife it's a fireman's pole unless she is willing to participate.