A scale model represents a ratio. All sides must be shrunk or increased based on a constant value. In this case the ratio of the model is 1/13 the size of the original giving us a 1:13 ratio. So each side of your scale model must be multiplied by 13 to find the real value of the side.
First convert all units to inches then divide the width of the real windmill by the width of the scale model (both in inches) you will see the answer is 13. Multiply the inch values of all sides of your model by 13 and this gives you the proportional value of each side of the real windmill in relation to the scale model.
