What paper format and font should I use for a book?

Nope. You’re thinking of font sizes. Courier 10 on a typewriter was defined as 10 characters per inch; Courier 12 was 12 characters per inch.

All us old-timers had to relearn this standard practice when computers came in.

Huh. Well, I’ve learned something today! Also, those are some nice fonts, I’ll have to keep in mind for the future.