Open up several books and you'll see that they are printed in a variety of font sizes. Some are meant to be oversize, either, to make the print easier to read, or, in some cases, to fill out more pages of the book. This choice is strictly up to the publisher.
In fact, the author RARELY has any say in this decision. When the writer turns in a manuscript, it is, generally, in a digtal form, such as in a Microsoft Word document, and the font is often a standard one that the book editors like for THEIR ease of reading. Then, if it is to be published, the digital file is turned over to the designers and typesetters and a font for the final book is chosen.
As far as what particular font will make it into the published edition, there are MANY classic fonts used, such as Times, Garamond, Bookman and several others. It will almost ALWAYS be a serifed font because, in a page full of text, such as that of one of your chapter books, a serifed font is easier for the eyes to follow, line by line.
Font families that are NEVER used are "decorative" ones. Like those that try to convey a theme, or those with extra strokes that have no purpose other than to decorate a letter. As a chapter book is read, the reader has no time to appreciate a decorative font and will be distracted by such letters.
Designer, Illustrator and Desktop Publisher for over 30 years