Normally 1600x1200 would do just fine. This gives you the option to continue using the wallpaper even if you upgrade your monitor size. More often than nott people won't even use 1600x1200 even if their monitor supports it, so you'll be pretty safe there.
tbob: at my office, I've a TFT Philips 17". At home, I prefer my CRT Iiyama A702HT 1280x1024 at 85 Hz. For me, a good CRT is really better for the colors than a cheap TFT at the same price... ;-)
I have a 17" Benq FP767 ver.2 TFT monitor. On my old windows computer I have a hewlett packard mx70 15" CRT monitor. Currently run them both on 1024x768, though they support higher.
"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." Bertrand Russell
I use LaCie Photonvision 20 TFT's with Pantone Spyder Pro Calibrators - get great colour and definition on them and they are half the price now they were when i bought them a year ago. The Spyders are very easy to use and have the annoying habit of messing up your screen colours when you restart as a non too subtle reminder if you seriously overun calibration intervals.
The best screensize to download if your going to be using it as desktop wallpaper would be whatever size your desktop resolution is set to in Windows. For example, if your resolution for your "Display" in Windows is 1024x768 then download a wallpaper that is 1024x768. I've noticed that if you use a wallpaper that is a higher resolution than your desktop is set at then you will get tearing and pixelation around the borders of the image. I like to download the biggest pictures possible then resize them to match my desktop(Which is 1280x1024). I also set my wallpapers to "Stretch" so it fits the whole image on my Windows desktop. I hope this helps!