For starters, if possible, I think rendering them in a ratio like 16:9 or 16:10 would be best. I assume most people will be able to use it as a wallpaper at one of these ratios.
For resolution I would say at least 1920x1080. When it comes to how high, I think it really just depends on how powerful your computer is, and how long you are willing to wait for the render to finish. Basically: as high as you can tolerate.
So far I've only rendered in Apophysis. Due to computer limitations, I rarely go above 1920x1200.