What size font should I use?

I'm writing a story for myself for fun because I love to write. I want it to look like a real chapter book. But I don't know what size font and what font to use in my book. Does anyone know what authors usually use in their books?
Update: Also, what type of font. Is it Times New Roman?
Update 2: This is just a hobby. I'm only 14 so it's not like anything major. I just love to type and write.
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