The resolution is measured by the amount of pixels, measured vertically times the number of pixels horizontally. The standard is pretty much 1080x1920 (or 1080p). I wouldn't buy anything less than that. The resolution is not directly related to the size.
there are quite a few factors to consider when buying a monitor.
size- 22" or 24" is most comon
resolution- 1920x1080 is most common
backlight type- ccfl or led. leds save money but can cause a blue hue
stand type- pivot, rotating do you need vesa mount for wall rack
panel type- tn, va, pva, e-ips, h-ips, p-ips, s-ips. tn has poor viewing angles.
brightness- minimums and maximums. typically you dont have to worry about this.
contrast- typically you dont have to worry much about this.
viewing angles- tn is a few degrees. ips type panels are almost 180 degrees.
color accuracy- is equal except for 8bit ips panels (s/p/h-ips) which are better.
refresh rate- 60hz is standard but 120hz is available on some tn panels. you might be on the 50hz/100hz standard though.
response time- tn panels have the least but this isnt a huge issue on any monitors nowdays.
what fits you best is determined by what you need the monitor for.