I built my garage before I built my house, priorities being priorities, in 1980.
If I had it all over to do again I would:
Make it at least 28 feet deep.
Doors configured so you can get any car out anytime you want. If you have to move something first, that car just never gets used.
If local zoning allows, make at least 1 bay high enough( 14' under ceiling) to accomdate a lift. I would consider making the entire building 16' under the ceiling, makes for many possible configurations with mezzanines up top for stuff ( Bradys garage was a good example). Foot print can also be smaller if that is a consideration.
Because of our peculiar vehicles, make doors 10' high, consider at least one door 14 ' high, for potential resale, a person with an RV may want to put it inside.
This company in my area has a stellar reputation, this is the building I have:
http://www.kistlerbuildings.com/ The owner is a neighbor of mine, and has been a member of our local school board for years...
Lots of power and outlets, don't forget compressed air, depending on how much work vs storage is going to be done in the building.
Enjoy the planning, it is really a fun part of the hobby... Jeff Beyer