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Exit pupil is the size of the image formed behind the eyepiece that enters the eye.
It ranges from the size of your dark adapted pupil on the low power end to about 0.5mm on the extremely high power end. Exit pupils larger than the size of your pupil get vignetted by the iris in the eye (you are stopping down the diameter of the scope) and exit pupils on the small end can be intercepted by floaters in the eye (you can see them on the Moon at high powers).
So there is a range.
The size of the exit pupil determines the brightness of the overall field seen--larger is brighter.
The diameter of your pupil determines the maximum brightness of the field seen.
Astigmatism in the eye affects all observers with maximum pupil. We see the stars with points on them. Without astigmatism, we'd all see the stars as tiny points.
As the exit pupil gets smaller, astigmatism bothers the image less and less.
At some point most people notice the image no longer has any astigmatism in the star images as the exit pupil gets smaller. From that point and smaller, glasses or optical correction are not needed. Hyperopia or myopia can simply be focused away, so only astigmatism requires glasses to see the stars as points.
You mention a 30mm APM eyepiece. I see horrible star images without glasses, but tiny pinpoints with them. That eyepiece yields too large an exit pupil for me to see an unastigmatic image. But when I reach an 8mm eyepiece, I see no astigmatism at all, without glasses.
My astigmatism is minor, though, and so I can dispense with glasses at an exit pupil of about 1.7mm. People with worse astigmatism may need to wear glasses at all powers.
2-3mm of exit pupil is often the point of best acuity for the observer (corrected vision, that is), so the image quality in that range is usually great. It's small enough to avoid a lot of large pupil astigmatism, and large enough that floaters in the eye don't interfere with the image.
Once you get to about a 1mm exit pupil, the Airy disc resolves. From that point down (higher powers) the star images get larger but no further improvement in resolution occurs.
However, the image gets larger so smaller details become easier to see, and the background sky darkens, so fainter stars become visible.
Exit pupils below 0.5mm get exceedingly dark (think how little light is entering the eye), so are usually not valuable to use. Also, seeing conditions usually prohibit such tiny exit pupils to be useful because of high magnifications. In fact, many people max out their magnifications with about a 1mm exit pupil (the focal length of the eyepiece matches the f/ratio of the scope).
I usually describe low power as 4 to 10x/inch of aperture, medium power as 10-20x/inch, and high power as 20-30x/inch.. Above that is the realm of ultra high power and it can be occasionally used, but not that often.
In exit pupil terms, that's 6mm to 2.5mm, 2.5mm to 1.3mm, and 1.3mm to 0.8mm. The 0.8mm to 0.5mm is the realm of extreme high powers.
I don't know what more to say except that exit pupil choice is only one of many ways to pick a set of eyepieces. Personally, I prefer to do it by picking the magnifications that work well in my scope.
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