A cupola too small looks like a birdhouse. A cupola too big overpowers the
building.
Measure the unbroken length of the roof ridge. You can lay the tape on the
ground.
Take the number of feet in the ridge. The optimum cupola is one size bigger
than this measurement, in inches. For example, a 12 foot long building with a 12 foot long ridge would use a cupola with a 16" base.