This page contains a calculator and a converter, which may be helpful in determining the dimensions of a model in a chosen scale.
Enter the width and height of the real-size car model, in whatever measurement unit you want. Enter the scale you would like the model to be printed in. For instance, enter "87" for HO gauge. Press the OK button. The width and height fields now display the dimensions of the model in the chosen scale, using the initial measurement unit.
You can also convert values from metres to inches and from inches to centimetres by clicking the appropriate buttons. To clear the width and height fields, press the C button.