24' x 24' is a good minimum 2 car garage dimension for parking.It leaves a enough room to have a workbench along the back wall.Figure a two lane road is a minimum 20' to have a centerline and you`ll want room to walk around the vehicles and open the doors.I`d want 24' x 30' minimum.Don`t forget all the shelves and junk you`ll have along the walls.
Seriously,you`l have the floor jack,jack stands,etc,that will need a place in there.