The old "rule of thumb" is:
'For every foot of building width, you should have at least one inch of cupola', or, 'One foot equals one inch'.
So, measure the width of your building and convert the footage directly to inches.

This guideline is a minimum rule of thumb to get a cupola into proportion.
1.25" per foot is also acceptable especially for buildings 2 stories or more.
Note: Pictured in the Drawing below is using the 1.25" per foot rule.

Some Examples of Cupolas on Various Size Buildings: Note: click images to enlarge photos
22" Cupola	24" Cupola	30" Cupola
30" Cupola	36" Cupola	36" Cupola
36" Cupola	36" Cupola	36" Cupola
42" Cupola	42" Cupola	72" Cupola