if you go with a lower gear ratio, you'll need to spin fast down hills, so go with a slightly shorter crank. if you go with a higher gear ratio, go for a slightly longer crank to give more leverage. with longer cranks, you run the risk of pedal strike unless your BB is sufficiently high.

I like ~75-80 gear inches as an all-purpose ratio. If you don't mind fast spinning, go lower. Flip-flop fix-fix hubs are good for flexibility in grocery-getting/barhopping vs training/racing/going fast