Measure the length of roof where you are planning to place cupola.
If you are planning to mount the cupola in the center of the roof of house, then, measure the entire unbroken roofline. Otherwise, measure the unbroken portion where you are planning to mount the cupola. Look at the unbroken roofline given in the diagram given below:
For large size, building you should hire a professional architect or structural engineer so that they could suggest you the size according to the building.
Irrespective of the size of buildings, cupola can be a focal point. However, for larger buildings, barns, the size of roof should have more than cupola. It will really make your building exterior awesome. For example, 60 feet roofline can have three cupolas. Therefore, you can place one larger cupola at the center and other two small size cupolas on the sides.
Look at the diagram given below: