https://www.bikecalc.com/gear_inches

70" is a good place to start. You can move up or down from there depending on your preference. More inches is harder to pedal/you go farther with each pedal revolution. I rode a hilly 1,200k last year with a guy who was running 85" (48x15), which is the tallest fixed gear I've ever seen someone run on a brevet. I normally run right around 70", a little lower early in the season, a little higher when I'm in better shape.