you can control this with .css as yihui offers
at the top level of your bookdown project there is a file called style.css open it and paste:
.book .book-body .page-wrapper .page-inner section.normal h1{font-size:1em}
this will increase the font size of the top-level title when creating the book. you can change the value of "em", or you can use other methods to determine the font size, such as "%" or "pt". you can also change h1 to h2, h3 etc. to change the headers of the second, third level, etc.
I recommend reading a little .css
network style like sewage
roman source share