You are actually asking about the length of a piece of string. The size will vary according to the target being used, you personal "preference", shooting position and light conditions. That's before we start delving into ring thickness, colour or shape.
As a starting point, using the NSRA 89 Series targets you would normally expect the foresight size in the prone position to be in the range 3.6 - 4.0. It could be a little smaller or considerably bigger. For kneeling it would probably be .2 to .4 larger than for prone and the range for standing 4.0 upwards.
As far as elements are concerned, the Centra Hi-End Glass with a 1.3 ring thickness gives an excellent image. Don't bother about different shapes unless you have a really good reason to use one,
Rutty