It's the width of the shell, 1/10 scale 'on road' cars come in 200mm and 235mm width (with a few odd exceptions) simply measure the width of your car to decide if a 200mm bodyshell is wide enough.
Be careful as there are also bodies in 1:10 scale at 190mm which is the standard for touring cars these days. As the post above states, it is the width of the car.
