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 exp
I won't consider only the desktop monitor resolutions. Nowadays you can browse websites on almost any device, and is fairly common for people to browse also with their mobile phones and tablet PC, so you really should consider them both.
This could result in a different site version for different devices, or a site layout that is ableto accomodate based on different sizes. This is something you have to consider when first thinking about the website you are planning to do, since not every website is suited to have a layout of this type.
An adaptable layout has the added benefits that it will be good even if new devices with custom (non standard) resolutions came out, you won't have to worry about checking it in those new devices because it will fit ok (if the layout was done right).
Anyway my standard minimun width is still 960px for desktop monitors, and for an adaptable layout I usually choose a max-width for really huge monitors (think about viewing a site which extends for the entire monitor width on a 2500x1600 monitor... gosh) of about 1200px, and a 100% width if the browser size is less than that, to allow the website to fit good everywhere.
960 is a standard for fixed-width websites. It is to ensure that 1024x768 resolution will pick it up easily. As for height - it does not really matter.
960px width is a good standard, as others have already elaborated on.
However, one should keep in mind that websites are viewed on a lot more than just desktops these days, so the answer could vary based on what the site is being styled for. For example, 960px would be a poor width for a mobile stylesheet.
One resource that could help you is www.resizemybrowser.com. (There is a similar tool built into Firebug.)
At the end of the day, I think 960px is a good standard, but really, "it depends". :)
In general, for a fixed-width website, that is meant to be read on a computer screen, nowadays, I'd say around 1000px (because there's no 800*600 resolution anymore and the least you can find is 1024*768).
You should really aim for an extensible design (generally harder to do though).
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.
Out of experience, for the web i would say a width greater than 850 and less than 1000 px preferably in the 900's and a height of around 750 px, but bear in mind that the height would be variable in case of dynamic content. so the height could be set to 100% or auto.
and best to align the website in the center, via the css property margin: 0 auto;