The average square feet of a 2 car garage is 676 square feet and is made just big enough to hold a couple cars or other vehicles as well as some tools.
Some people use their car garages to store other things instead of their vehicles but the best use of a garage is for a vehicle.
The average square feet of a single car garage is around 350 square feet and the average square feet of a 4 car garage is around 1,440 square feet which is about the size of an average house.
The larger you make your garage the better because you'll have more room to store tools and work on your vehicle instead of just parking your vehicle in the garage.
I have a 4 car garage and although I don't park 4 vehicles in it I wanted enough room to work on my vehicles in the garage and work on other things as well.