5:1 would be very high! Sizing will depend on the machine - what are you trying to drive? My small Denford mill has a 2:1 reduction and is quite happy with small type 23 steppers. My Myford lathe has a larger, still type 23 stepper, but direct drive on the X-slide and I think 2:1 on the leadscrew IIRC.
Better resolution comes from using microstepping on the drivers. On the mill, the screw pitch is 5mm, so one full step is 5/(2x200) = .0125 mm. Then I use 8 microsteps per step, so the theoretical resolution is .0125/8 = ~1.6 micron.