You can never have too much garage space.
I agree that the 24 is a bit short. My garage is 26 deep by 36 wide (10 foot ceiling) and is fine for short cars like Corvettes, but if you plan to put a 1970 Cadillac in there you better go longer. I did not build the garage, but for the yard space available I would have gone 32 deep by 52 wide (12 foot ceiling).
For winter storage I get four cars in side by side even though one bay has a two post hoist. It is a three door garage, basically three 12 foot wide spots.
I like the 9 foot doors as well, but they are not so convenient if you are having to put a vehicle in on an angel. But that is where the floor jack comes in handy, just lift it up and roll it over to the side.
Basically the ideal setup is your clean parking garge attached to the front of the house, and a separate working garage out back, that has walls to provide physically separated "clean" and "dirty" areas. And of course there should be a bathroom and shower, fridge, tv (to keep up with the car races), microwave, stereo a few chairs and table, library space for all the books, computer for handy reference.
It is nice to have wall electric outlets about every six feet and between the doors, just about everywhere. Good flourescent lighting. A couple of hoists.
Security system...........etc, etc, etc..........and of course tools