I am not sure i really understand your question, but i think i can give it a shot.
http://world.guns.ru/rifle/rfl19-e.htm
that link should take you to a page showing a P14 rifle.
the bottem pic should show the diopter rear sight your asking about.
"what size black circle, and how many yards"
I assume your askign about a target? use whatever you have readily available, as for range, if you can find a 100 yard range use it, if you can only get 25, you want to set the sights so that usign the 100 yards range marker, you hit 3 inches higher than where you are aiming, at 25 yards.
that is fairly standard for _most_ .30 cal rifles, and will give you a max point blank sighin of roughly 300 yards in an 8 inch circle.
That is assuming rifle is in good condition, and everything is working properly.
what that means is, set that way, aiming center of mass, the bullet will never go above or below that 8 inch circle, until sometime after roughly 320 yards or so.
8 inch cirlce is generally consiered to be the killzone on a deer, and is just a standerd rule of thumb.
if this isd greek to you, or i misunderstood, hollar back