An important foundation for any publication is a Style Guide or Style Manual. Some might suggest: first get your written content captured – then worry about the style. But it is important to have some rules laid out as to how you will display Headers, Footers, Mirror Image (left and right pages), page numbers, and content. Personally, I always appreciated what Michele Weiss did for me back in 2001. I found how important readability is, and how easy it is to drift away from a consistent look, style, even the font size and type as you add more chapters. A Style Guide reminds you of those chosen parameters. If you want to change them, fine – it is your book! However you need to make that a conscious decision, not just do this for awhile, and then that.
I have linked to a two-page Style Manual that I used for my first genealogy book – yours may, and probably should, be different, but the point is to establish consistency. The examples linked below contain two confusing acronyms that were meaningful only to my project: MO was just the name of the publishing project; and FO was Family Origins, my genealogy software program (which has since evolved to RootsMagic).
The style guide references a “major descendant.” That is just the primary progenitor for a chapter – your project perhaps has chapters named for people; if so, those are the ‘major descendants’ – and in my book I wanted to distinguish them by larger font as shown in the Style Manual.
Finally, it is important to note that all fonts are not the same “size” – thus 11 pt Calibri appears smaller to the reader than 11 pt Arial or 11 pt Helvetica. Try out a few ideas, and then capture ‘the best’ in your Style Guide.