I write design docs frequently. They can be anything from two to about ten pages, outside of formal systems like aircraft. They need to be short to be readable.

I write design docs frequently. They can be anything from two to about ten pages, outside of formal systems like aircraft. They need to be short to be readable. The point of a design doc is to get feedback on the design and to keep a record of decisions made and why they were made. That’s it. It isn’t supposed to stay up to date forever - your code should document that instead. Once you get feedback on the design, the doc has served its purpose - archive it and move on.