Sizing my Cupola

The old "rule of thumb" is:
'For every foot of building width, you should have at least 1.25 inches of cupola'. 
So, measure the width of your building and times that by 1.25. 

This guideline is a minimum rule of thumb to get a cupola into proportion. Structures two stories and higher may require one size up, or 1.5" per foot of roof.