As others have said, there is no "rule". But I once took a class from Barb Vlack, she said you can use the Fibonacci Sequence as a guide to help figure out proportions.
The human eye finds this sequence pleasing because it occurs a lot in nature.
Here are some examples, (I especially liked the nautilus.)
Jinny Beyer wrote an article about how you can apply it to quilting.
http://www.jinnybeyer.com/blog/fibon...-golden-ratio/