The size of a telescope, two inch versus a larger aperture, determines how dim of an object you can see. For example, my 8″ dob rarely sees anything dimmer than 8 magnitude. The 24″ at the observatory can see objects between 13 and 14 magnitude on a dark night.
Making an object larger in view has to do with eyepieces. The larger the mm, the larger the field of view, and the smaller the object will appear. These eyepieces are necessary when hunting down an object (for purists like me who prefer no computer on the telescope) or when viewing a large object. For example, viewing the Andromeda galaxy and its two companion galaxies.
The smaller the mm on an eyepiece, the narrower the field of view, and it will make the object appear larger.
So, the first consideration, obviously, is budget. Small telescopes are usually reasonably priced, so are Dobsonian telescopes without computerized goto systems on them.
The next thing to consider is where you’ll be using the telescope and how. Weight becomes an issue if you have to carry a telescope out into the wilderness or somewhere. Ease of set up becomes an issue, too.
The easier a telescope is to get out, lug around and set up, the most likely you are to use it often.
Size is another consideration. Will it fit in the car? Where will you store it?
One way to try out telescopes and eyepieces is to visit local star parties. Most cities have a group of amateur astronomers who get together now and then to get out and view. Or, if you’re fortunate enough to have an observatory nearby, go visit. I’m really fortunate here as there are two observatories nearby. The more you know, the happier you’ll be with your decision.
To me, the biggest factor was ease of use and portability. I’ve been very happy with my 8″ inch Dob. It gets clear views and with a nice, expensive eyepiece [an 11 mm Nagler], its views of globular clusters rival the 24″ observatory scope.
I’ve done some photography with it with my little Sony Cybershot. Mostly the Moon, Jupiter, Venus and Saturn. Some day I may get more seriously into astrophotography, but for now, I’m very happy with the telescope I chose.