After brainstorming and before writing your outlines, write your summaries first.
Having a basic storyline in place before outlining and drafting helps to keep you on point throughout the book. This way, if you decide you want to change something while you are outlining or drafting, you can look back at your summary and make sure the storyline will still make sense.
There is no specific length for a summary, it’s for your own personal use. Personally, I start from the beginning of the story and include the major plot points that get my character or idea from beginning to end. The little details are important but they shouldn’t change the major plot points in the long run.
Writing your summaries first gives you a baseline to follow for your whole project. It covers the highlights and key points. Save the details for your manuscript.