What size font (typeface) you use depends upon the type of book you are writing. The size of the font should correlate with the reading ability of your audience. In a novel for young adults and up you can get away with a 10.5 at the smallest. You probably don’t want to go larger than a 12 unless you are writing specifically for geriatric readers, in that case, you may want to go up to a 13. In picture books you don’t want to be smaller than a 16.
The smaller your font size, the more words you can fit on a page. The more words on a page the less pages you have. The less pages, the cheaper it is to print. The cheaper it is to print, the lower the cost is for your readers. The lower the cost, the more books you sell. The more books you sell, more royalties you get. You get the point.
The style of font will impact the size of font you choose. Some styles are just bigger or smaller than others. If you choose something with flare, it may need to be larger to make it more readable.
Speaking of flare in your font style, it is not recommended. Go with simple and professional fonts. Less curves is better. Really the only time you should be diverting from this is for your title, chapter headings, and to make an example of font style. Okay, okay you can get away with more fun styles in children’s picture books. Even in picture books they have to be easy to read. The audience is trying to learn their letters, so they should be able to easily recognize them in uppercase and lowercase.
Type up a paragraph or page (using 1.2 or 1.3 line spacing) and try reading it as you would a book. As other people in the age group of your audience to read it. Is it easy to make out the words? do you have to hold it closer? are their particular letters that are more difficult to make out?
If you are seriously stuck on a font style and there is some difficulty reading it, try increasing the space between letters (google how to do this in the word processing program you use).
Of course people have researched what size font is best for various ages. Here are some general guidelines based on age of the reader:
|Age||Type Size||characters per line|
|7 to 8||18||38|
|8 to 9||16||45|
|9 to 10||14||52|
|10 to 12||12||58|
Happy and productive writing and editing.