What is the best width for a post front sight, to be used with a rear peep site? Is it the same if the primary purpose is for hunting rather than target shooting?

Wink, hard or easy to answer, depending on your perspective. Target pistols have a front sight that is the same width as the black on the standard 25 (and 50) yard targets. Any narrower or wider makes it much harder to ensure that the front sight is centered on the black. Obviously with the 6 o'clock hold that most bullseye shooters use, the POI is higher than the POA. Presumably for hunting you could change this so that the two are the same.
Hope this helps!
Peter.

The reason I ask the question is that I put a NECG rear peep sight on my CZ 55O and a front post sight (patridge/sourdough). At 50 meters I can put three shots touching each other. At 100 meters I am all over the place, mostly, I think, because my eyes can't accurately place the post. I wondered if a thinner post might improve my accuracy.

Wink, in line with my initial response, it might be that your aiming point (the black) is not subtending the same angle as your aiming point at 50 yards. Try a 100 yard rifle target and see what happens. Alternatively try using a large black square (again this should be the same "size" as your front sight) and use a 6 o/clock hold. This should give you the accuracy, but may not translate to useful hunting accuracy. Hunting with a flat front sight at long ranges is a little iffy anyway. I would suggest using a hi viz sight of some kind. Probably not the most accurate, but if you are shooting a magnum caliber of some kind it should not matter that much. Hope this helps.
Peter.

Wink,
Size of the front sight depends on your eye sight, barrel length and the purpose intended.

If the sight is on a back up rifle in bear country you might want it to be very large. If you are trying to shoot squirrels out of tree tops with a .22 you may need a very narrow sight.

The previously mentioned posts talking about the target width compared to the sight width are very good. Mauser front sights are usualy difficult to hold elevation with. However use a target shaped like a letter "A" or upside down "V" and they work great.
One other thing that helps with targets.
Use a LARGE white backing (say 18" X 18" )with a black target. The white back ground helps the sight stand out.

ireload2, the NECG provided post front sight is 0.99 inches wide. Around a tenth of an inch. What are other typical widths one sees for front sights?