Writing Books/What size type is used in...
What size type is used in a hardcover book and how many words to a page?
I will wrinting on a computer. What size font should I use?
We do not know what font or size you should use because we have no idea what genre it is or how lengthy it is or what trim it's in or who your audience even is.
You will need to let a professional illustrator decide this for you before it goes to print. In the meantime, just go to work with it.