First, note that a house style guide is not a comprehensive compendium; it is a resource that details rules and guidelines for consistency that contradict or are not covered in other resources. That said, a house style guide can range from a couple of pages to more than a thousand. (The Chicago Manual of Style, which exceeds that latter number, is simply a house style guide that escaped from the University of Chicago Press, for which it was created more than a hundred years ago, and went viral.) The size depends on how often, and how significantly, your publication veers from default resources and how complex its subject matter is. (from Daily Writing Tips)
The above from Daily Writing Tips is about creating your own style guide. To read more, click the hyperlinked text above.