Question

A couple of years ago my son showed an interest in astronomy and we bought a 6" reflector telescope. We use it pretty regularly and have enjoyed it immensely. Lately we've both been wishing we had something bigger to be able see more things and to see what we can see now with more detail.

How do you determine the size of telescope needed to view a certain object?

I understand that there are a lot of other factors that come into play when talking about what you can see and how well you can see it. Ideally I suppose what I'm looking for is some sort of chart/table that gives a general guideline of the scope size and some of the objects that should be viewable (with an average setup).

Are there any such resources?

Answer 1

There are lots of mathematical answers to this question, but I'd like to make a few qualitative observations instead, based on 54 years using telescopes of all kinds and sizes, from 40mm refractors to 74-inch reflectors.

Unless you have some specialized purpose, don't consider anything smaller than 6 inches aperture. Small telescopes look cute, but don't show you much, especially if you're a beginner. Experienced observers can tease amazing observations out of tiny scopes, but most of us will be happier to give these a pass. Aperture wins.
A 10-inch Newtonian on a Dobsonian mount is something of a "sweet spot." It's about the smallest aperture to show significant detail in deep sky objects, yet is compact and light enough to be easily transported to dark sky sites.
Above 10-inches, the more aperture the better, provided you can comfortably transport, set up, and operate it. This is crucial! The nicest telescope in the world is useless if it never gets used. I find even a 12-inch Dob becomes bulky, cumbersome, and heavy. Aperture wins, but only if you use it.