Page length isn't a factor for where Google will rank the page. There are many sites that rank with short pages and plenty of long pages that don't rank. What you are probably thinking of, and where you run into trouble, is thin content. What @closetnoc said is right, that you want to focus on the completeness and thoroughness of your content.
So, as you think about writing your manual, how many words would it take to completely and thoroughly answer the reader's question? For some subjects, that will require a lot of words. And a lot of words doesn't mean bad UX. You can organize the content with many subheaders and subsections on one page. For other subjects your manual covers, a short paragraph might be enough.
And - remember that it isn't just the words on the page either. Some pages might have few words, but a long video or an image. Those pages can rank and work great for SEO. It is all about answering the question people are trying to answer. So write a page that answers the question better than anybody else and you'll increase your chances for that page ranking.