What are the most commonly used pixel sizes (primarily widths) and what are there advantages and disadvantages?
How can I best find a happy medium to give a good experience to people with a wide variety of monitor sizes?
An answer with an explanation rather than just the size would be greatly appreciated.
the most grids use a width of 960px. (When the design has a fixed width). When you take a look at global statistics 1024 range resolutions are still the most common: http://gs.statcounter.com/#resolution-ww-monthly-201006-201106
Do not use 1000 width. (You have to count in the border width of the browser and the scrollbar, in certain browsers / OS they are larger then in others)
I don't think there is a ultimate resolution that's why you should check the statistics on the concerned webpage (if the page already exists), to decide what resolution is most appropriate. If you can't do this you can check stats for you target market on http://gs.statcounter.com/
Or even better use responsive webdesign: http://www.alistapart.com/articles/responsive-web-design/
Since max- and min-width attributes in CSS, you can target whatever resolution you want, if you have the time / budget for it of course.