What is the proper size for a cupola?

This is a design question that defies a simple answer. 

The answer depends on a number of variables.

Aesthetic appeal in terms of the cupola's proportional relationship with its surroundings is of course extremely important.

One should also consider: what supports the cupola, how much it weighs, its intended purpose (i.e. as lookout, lantern, ventilation, or simply ornament), and any height or code restrictions that govern its construction. 

How it will be built and located in its final position can also affect the answer to this question. 

Overall it’s a basic rule of proportion and taste.