Screen resolution depends entirely on the monitor they have, it varies widely. 

To find out what your current resolution is in Windows right click on the desktop and choose properties, then under the settings tab the screen resolution is listed and can be changed.  Examples of when you'd need to change the resolution include a new monitor, plugging in a projector, or simply changing the appearance and size of everything displayed.

the resolution is listed in pixels and generally 100 pixels equal an inch at a normal resolution of 96 DPI.  The resolution is as important as the screen size and can usually be found under the advanced button of the above tab.

I hope this helps, as long as you match up their resolution (DPI) to their screen size(eg. 1280x800) you'll be fine.