I'm not positive about the diameter of the stock sight, so I'll leave that to another TFL member.
There is really a lot to say about NM rear sights, bases, windage knobs, etc., so I'll keep it simple.
There are two sizes of NM rear apertures. One has a .0520 peep hole and the other a .0595. Both are hooded so you'll get a more consistant sight picture, and the peep hole in both is .002" vertically eccentric (offset) with the eyepiece diameter.
That way you can rotate the eyepiece to raise and lower your line of sight.
I've got the .0520 on mine, and believe it or not, the NM sights really will help tighten those groups!