Our signage products are all customized, so we don't have any standard size.
Mostly we refer to size of a business signs as the "total width", which would be call as "overall length" too, such as 1000mm width, 6 feet long, etc. We will scale the height based on that, proportional to your business signs design file
It is also a matter of personal preference. Some clients want the signage to be larger and more prominent. Others want it smaller, and more subtle.
Base our experience, the golden ratio(0.618) can help you have a very good business signage. For example, if your wall is 1000m wide, a 620mm wide signage be placed at the center of the wall would look amazing.