I suppose a more specific approach to what I'm trying to find out is; how and what factors would help me in determining what size eyepiece to purchase?

As it stands now i have a very respectable 10"schmidt-newtonian, but I only have a Meade Series 4000 25mm Plossl and a 2x Apochromatic Barlow Lens. I can't figure what the next best step in EP should be, I live about 15miles S of Denver, so moderate light pollution, but I don't know if that would be a factor in deciding between say a 6mm or 12.5mm EP.

I am also wondering how to determine how exactly the whole magnification factor affects my viewing ability/limits? Does that make sense? Is there some magic formula providing a link between magnification and, say, magnitude?

**To simplify my rambling nonsense, how to you figure what size EP you need to see "X" object, and what needs to be factored in?**Thanks for clearing this up guys!