there is no "perfect" size as it applies to the space it needs to fit in; and this varies. In a typical modern two car garage (fits 2 Honda Civics) you probably won't have room for anything more than a 20" deep box on the side wall if you want to be open your passenger car door. Or actually want to remove a tire on that side of the car without feeling constrained. If you have a monster garage, then you can go as big as your budget allows.
I have older Craftsman boxes and they are 18"? deep and that is all I have room for on the sides of my 2 car standard home garage. Yes, I could have 24" deep ones and then barely be able to walk between the box and the vehicle. As it is, when I am going to do any auto maintenance, I park the vehicle towards the center of the garage, barely fitting in that side of the door, and then very carefully back it out.