Mines 26x36, and not large enough. And that is moving from a house with a single car carport converted to a garage.
I now also have a two car attached garage that I've filled. I've got five "bays" total, four vehicles, and yet two live outside now. To my credit, three of those four are full size cars/trucks. Athough I'm less than organized, when you add in the riding mower(s), project car, project truck, trailer, arc/mig/oxy/acetylene welders, bandsaw, drillpress, work bench, parts washer, table saw, spare cases of various petrochemicals, router, rolling tool box, engine hoist, floor jack, jackstands, compressor, fridge, spare wood and steel, cabinets, shelving, spare trans and t-case(s), motor(s), etc. that you invariably "need" to do your own projects, it gets cramped way too fast. That's not including all the spare parts I've put up in the rafters for when you can't get stuff in the wrecking yards anymore. A sink with hot water would be nice too. If you can't clean your hands off and take your shoes off, and you work on stuff as filthy as I do, you'll bring it in the house with you. At least with the detached, the grease tends to stay out there. No one complains about grease fingerprints on the door.
A true two story shop would be far more functional, with a stairwell so that storing and accessing things up there is easier. Still going to end up with the heavy stuff on the ground floor. Detached also isolates some of the noise from grinding, cutting, etc. that might be objectionable if someone was in the same building trying to sleep, watch a movie, etc. Smell from chemicals is another factor.
I did think intially that having a detached garage would be problematic/annoying, but other than being cold (not done insulating it yet) in the winter, I prefer it that way now. Additionally, since the detached garage doesn't need natural sunlight like a living space (IMO) it is pretty well buried in the trees, which keeps it far cooler than the house in the summer.
If you are an immaculate/anal person when it comes to organization, an attached shop would probably be ok. But there are times that things need laid out (like rebuilding a transmission) and there is no substitute for a flat, covered space.