IIRC, the default that comes is a .090'. If I am correct, that one is good for hunting in normal light. Can remove it for a .200" ghost ring (again, IIRC). If you want to shoot target/precision, then I think Andy has a .040" peep. Double check me on all these measurements, as I am doing this from memory.
I usually ship with the .096" aperture (because we always have) but I can tell you on my own I use the .155 out on the barrel. For the standard receiver mounted sights I do prefer the .096" for an all around choice.
Thanks OscarFlyTyer for the input! Correct, .200" with the aperture removed. (Which works great with the barrel mount)