The most common resolutions are (w x h): 800 x 600, 1024 x 768, 1280 x 960, 1600 x 1200, arranged by most to least common. Although 800 x 600 is still the most common resolution in use by the general public, 1024 x 768 and 1280 x 960 seem to be the more popular of those who comment regularly here.
As for sizes of certain elements, dabe gave you some sound advice when she suggested you take apart some skins you like and see what makes them tick. In the long run you'll learn more than you might expect by doing it that way, and the truth is, there are very few skinning tutorials out there that are current with the software versions available.
You mentioned Sysmetrix specifically, and that would be a good one to start with. Neither the graphics nor the coding is so complex that it will make you crazy, and there is a pretty good manual that comes with the software. The skins are easy to examine, they are all in your Sysmetrix/Themes folder. Take a look at the graphics with your program of choice, and open the .ini files with Notepad.