Most attached garages these days are too small. We had a house with a 20x22 garage called "2 car". It was a no car garage.
I suggest minimum of 24' deep, (28' better, 30' best) x 12' to 15' width for each space you want to park a car. So minimum 24' x 36' and ideal 30' x 45'. With a 30 foot depth you will have room in front of the cars for a work area or for tools, and with 15' for each car you will have room to open the doors and room between them for other stuff. If your lot space will allow it you will not be sorry you went for 30x45, and it will not cost that much more than a small 24 x 36 building.