Short version:
The reason that 2" eyepieces will offer more comfortable views is because they are lower powers and therefore have larger exit pupils.
Long version:
In a telescope, the objective is the lens or mirror which collects light and focuses it at a distance equal to the focal length, creating an image at that point.
(You can test this out most easily on the moon: leave the dust cap in your focuser and use your finder to point the telescope at the moon. You should be able to see a clear (after focusing!) image of the moon projected on the dust cap)
When you insert an eyepiece, you're using it to inspect the image created by the objective.
This is why eyepieces with smaller numbers actually magnify the image more: with a 5mm eyepiece you're looking at a smaller central portion (imagine 5mm, though that's not correct, as it also depends on the FoV of the eyepiece) of the image which the objective has collected; and with a 30mm eyepiece you're inspecting a larger part of the image.
And this results in the exit pupil - a function of telescope and eyepiece - which is the size of the image you're looking at. It's the beam of light which enters the pupil of your eye. At lower magnifications (higher eyepiece focal lengths) the exit pupil is larger, and therefore easier to view comfortably.
You can test this out: cut a series of small holes (from 0.5mm up to 7mm) in a piece of paper and try looking through them. You'll find that the larger holes are much easier to see things through.
To calculate the exit pupil, divide the focal length of the eyepiece by the F-ratio of your telescope.
(And, related, to calculate the magnification you'll get from an eyepiece, divide the focal length of your telescope (in mm) by the focal length of the eyepiece (in mm))
For example, your telescope is f/8, so a 30mm eyepiece would give an exit pupil of 3.75mm - nice and sufficiently wide for you to be able to find the view and not lose it.
But if you want to look at a planet then you'll want more magnification, and you'll have to use a shorter focal length eyepiece. Take a 5mm for instance. That would give an exit pupil of only 0.625mm.
Another effect of decreasing exit pupil is decreasing image brightness. You'll find that, apart from the brightest objects, you probably won't want to go below a 1mm exit pupil very often.
Here's an image from the Wikipedia article on the subject which nicely illustrates the concepts:
(Image credit: By Evan Mason - Own work, CC BY-SA 3.0)
You'll see another term in the image: eye relief.
Eye relief is the distance from the surface of the top lens of the eyepiece to the point where the image is formed (the point where you position your eye). This distance increases as the diameter of the lens increases.
If you wear glasses while observing then it's important to buy eyepieces with an eye relief of at least 15mm (actual figure varies, maybe ±5mm, depending on the shape of your face and glasses you wear) so that you can see the whole view.
(Also, if you wear glasses while observing, are you sure that you need to? Simple long- or short-sightedness can be corrected by focusing, and mild astigmatism isn't visible at higher powers.)
You ask
how can I choose a 2" eyepiece that would be more comfortable for
observing the Moon and planets? What should I look for in terms of
types or specifications?
The Moon and planets are objects which you'll generally want to use a high magnification to view. High magnifications are served by 1.25" eyepieces (there's no need to make them as large as 2" - they're looking at a smaller part of the objective image - and making them in 2" barrels costs more to make) and give a smaller exit pupil.
In your telescope, a good starter set of eyepieces, in my opinion, would be:
- A low power wide field eyepiece, with a focal length of 30-40mm, and a FoV of 68°-82°.
- A medium power wide field eyepiece, with a focal length of 10-20mm, and a FoV of 76°-100°
- A high power wide field eyepiece, with a focal length of 5-10mm, and a FoV of 76°-110° (This one's a bit more tricky, as the maximum magnification which will be usable will depend on your local conditions - the "seeing". If seeing is generally good you might be able to go higher than 5mm, but if it's bad then you might not be able to go much higher than 8 or 9mm)
(Notable brand names which come to mind are APM, Baader Morpheus, Explore Scientific, Pentax, Tele Vue)
You don't really need more than three good eyepieces.
You could also get away with only one high-power, an 8-24mm zoom, and a Barlow lens. But I think the wide field eyepieces are the better option in your telescope, because with a wider FoV you don't have to nudge the telescope so often to keep the object in view.
(The "good" is quite important: almost all eyepieces look good in the dead centre. Then the view starts to degrade towards the edges. Generally, the more money you pay, the less the view degrades (or starts to degrade further away from the centre). Also, and fortunately for you, the degradation of the view is greater as the F-number gets lower. At f/8, you won't need to spend loads of money to get great views, but you'll have to do your research and find out what other people with the same telescope like to use)
Finally, a couple of tips for comfortable viewing:
- Make sure you're sitting down while observing. It makes a huge difference to your ability to keep your head still. If you must stand, then try and have something to hold on to or lean against.
- Practice makes perfect: the more you use your telescope and eyepieces, the better you'll get at using them, and you'll find it easier to maintain a steady view.
These related posts Eyepiece diameter for small focal length planetary viewing on a large dobsonian? and What are the advantages and disadvantages of a 2-inch eyepiece versus a 1.25-inch eyepiece? have good answers to similar questions.
