Unlike a lot of telescopic subjects, the Moon is bright. That’s good news if you do most of your viewing under badly light-polluted conditions. Pristine dark skies don’t help one bit. (Besides, you can’t really have pristine dark skies when the Moon is up!) The lunar surface is also jam-packed with all kinds of fascinating and intricate detail. So for lunar exploration, you don’t necessarily need a big aperture to gather a lot of light, but you do want an instrument capable of good resolution. That means a small- or medium-sized telescope will work very well on the Moon, as long as it has quality optics.
When considering which telescope is best, most people put more emphasis on the instrument’s optical design than necessary. In the many, many years that I’ve been looking at the Moon, I’ve had excellent views through refractors, reflectors, Maksutovs and Schmidt-Cassegrains. Generally, it’s the aperture (the diameter of the objective lens or mirror) that makes the biggest difference between individual scopes.
That said, a few truisms do apply. For example, a 4-inch apochromatic refractor often performs better than a small Newtonian reflector. This usually isn’t because of shortcomings inherent in the reflector’s design but because of the fact that a 4-inch apochromatic refractor is a premium instrument, while a similarly sized Newtonian is almost always a budget scope that costs only a fraction as much. In optics, you generally get what you pay for. Another truism is that aperture rules. All other things being equal, a big telescope shows more detail than a small one, at least in theory.
However, theory and reality often part ways when we take into account the air above us. No matter how big or good your telescope’s optics, it’s the Earth’s atmosphere that ultimately limits the detail you’ll be able to see. It’s what astronomers are referring to when they talk about “seeing.” The air above us works like a weak, poor-quality lens. When the atmosphere is a turbulent, roiling mass, your telescopic views will be fuzzed out. In such situations, a big scope will not show much more than a small one. And the higher the magnification, the more apparent the effect. So to see the most on the Moon, you have to choose good optics and use them on nights when the seeing is steadiest.
Optics aside, you must also consider how the telescope is mounted. Because lunar observing often involves high magnifications, it’s very handy to have a mount with a motorized drive that will keep the lunar disc centred in the scope’s field of view by compensating for the Earth’s rotation. You’ll see more with a small driven scope than with a big non-driven one because you won’t be constantly fussing about trying to aim the instrument.
A final consideration is your viewing location. Can you observe from your driveway or backyard, or do you need to transport your telescope? If the latter, then portability becomes a consideration and you may have to sacrifice some aperture and performance for a scope that you can pack in the backseat or the trunk of your car. You should also be wary of purchasing a scope that’s heavier or bulkier than you can easily move by yourself.
Having said all that, I have a confession to make. I started my Moon-viewing life with a telescope that (a) was small, (b) did not have the best optics in the world and (c) rode on a fussy, nontracking altazimuth mount. In short, it was a scope that lacked all the desirable traits I have outlined above. And yet here I am, many years later, still enjoying the Moon. It’s worth keeping in mind that “good enough” is often easier to obtain than “perfect.”