Authors tend not to have much say into this :( It is sad but true. On the other hand it means the Publisher tends to know what works better. It would be horrid to have to read a book in Comic Sans.
It really depends on the size of the pages, the amount of pages, the writing style, the type of market they try to get the book in.
Some use big letters for easy reading with allot of whitespace for instance chicklits.
And others use smaller letters and much less whitespace to put more information on one page. For instance a study book or some kinda of medical resource book. The KJV has a particular lack of whitespace and uses a really tiny font.
There is not perfect font size and type. Different ones are used in different situations :)
Now for your manuscript many agents like the simplicity of Courier 12pt with Double Spacing. Because it is really easy to read! Also Double Spacing allows editors and proof readers enough space to make notes.
Once you get your book published a more fitting font is sought depending on what is said before.
If you are just writing stories and you want to make it look fancy while writing it go ahead. Just remember when you have to send it in as manuscript or send it in to editors/proofreaders expect to make it all ugly with Courier 12pt Doublespaced.