It all depends on how much you want to spend . Evne a cheap crappy 19" or 20" inch will set you back a good bit , but you can get top of the line 17" monitors for nothing. Of course a bigger monitor is beter, but the main thing is to buy a clean crisp one, If you can afford a good 19 ro 20, then buy it , but if not , it would be beter to go with a really nice 17"
The virge based video card you stated you have in another listing is not a good choice for any monitor past 17". If you want a monitor past this, you will need a good video card that is capable of the resolutions and high refresh rates these monitors support. Your virge card will not put out the refresh rates you will need to get flicker free performance on a 20" monitor. If you really want one though, and have the opportunity of getting one cheap, then you'll have to pop out you video card too and get something like a riva tnt or a matrox g-200. These cards have excellent support for larger monitors such as 20" ones. Refresh rates on both these cards are top notch. Refresh rates are the key to no headaches when using your computer. Always use the highest resolution your monitor/video card combo allows. Also, what is flicker free on a 17" monitor will likely not be on a 20". This goes for all size monitors.
I hade a standard type ultra vga 14" monitor that supported up 1024x768 resolution at 60hz. On that size monitor, the refresh rate at 60hz was ok. Now when I got my 15" monitor it visibly flickers and give me headaches at refresh rates below 72hz. 60hz was the standard refresh rate I used on my 14", but with my 15" after 5 minutes my eyes hurt and I get a headache if I set the refresh rates to 60hz. I had a problem with an app the other day that switched my resolution, and dropped the refresh rate down to 56hz. I almost freaked out, by eyes physically pounded in my head, and I could feel the pain in the top of my head looking at the screen. Needless to say, I ran to the display properties and changed the monitor refresh rate back as fast as I could.