well a compentent contractor should be able to match the existing house as far as roof pitch and roof and trim details..using siding to match the existing home..that way the garage looks like it belongs with the house..
As far as size we usually build 12X24 service bays as that is what is needed to fit in a car and a mechanics toolbench..
One of the more succesful ones I built was 24X36 two bay with shop space in the rear of the garage..this seemed to work out just fine and the guy I built it for was happy..
Personally I like two 9 ft doors better than one 16 ft door on the garage and of course a man door on the side or rear convenient to the house..
I have tried most all of it and now do what is known to work..