Hi Susan! Welcome to Nelsonfoto.
The one nice thing about a large monitor, especially with Photoshop, is that it allows you to have a lot of things open on your desk top AND viewable at the same time. In Photoshop, this includes the main screen with the images and then the other smaller windows with tools, history, actions, etc. I do most of my adjusting on a little 12" laptop, so many functions are a process of shifting things around in order to get to the thing I want, and then shifting things around to return to my original position. A big monitor lets you keep everything out and open, just as you might on a regular desk. Bigger is better for graphics.
An advantage LCDs have over CRTs (beyond radiation) is flicker. CRTs refresh their images like televisions do. If you have seen a still photograph of a television, you may remember seeing a band or half an image on the television. That's the refresh at work. However because it goes so fast, the effective view is bandless. But you do see the bands & the refresh. If you spend several hours adjusting photographs on a CRT, and then turn out the lights and open your eyes, the dark room will flicker just like the screen did a few minutes ago due to the latency effect in your eye/brain. I eventually found this bothersome (headaches - eyestrain) and switched to an LCD as a result.
Not all LCDs are made the same. I did some serious shopping for one a while back, and the one that was the absolute best in my estimation was the Apple Cinema Display (I was going to get a 20" because the larger ones were outside my budget.) There were other cheaper models by other manufacturers (Viewsonic for example) but the image on the screen & the color control was just not there. Apple really does make some of the best screens out there. I did find screens that matched Apple (LaCie) but they were either just as expensive or more.
I stopped looking at screens when the 24" IMac came out. I need a new desktop computer anyway, so I decided I would just limp along with my IBook until next year when I got a new computer (budgets being budgets) and buy a 24" IMac then. Bigger is better for graphics editing (Photoshop etc.) but your choice should be governed by budget and space on your desk. For me, the Apple 20" Cinema Display met the price ($699) image quality and space (very thin - important for NYC apartments!) requirements I had for a monitor, and I would have bought one, as I said, if the 24" IMac not come out.
Used - you want to be able to plug it in and look at it carefully. Look for dark lines or dark spots by opening a white page in say, a word processing program or something to see if there are any dead pixels (LCD) or banding problems (CRT). If you have an Apple laptop, you may need an adapter to connect the monitor to your laptop's port.
The best way (for me) to check monitors is to put several of your favorite images on a USB flash drive and carry it around as you look at different monitors. Plug it into the computer attached to the monitor and see what the images look like. Or if the monitor is connected to a computer that is also connected to the Internet, check your images on your Flickr page. That will give you some baseline of image rendering that you can use to judge the quality of the monitor for you.
Hope that helps!