Actually, we no longer develop according to pixel size any more. By using Responsive Web Design techniques, we can make the same page accessible across a wide variety of screen sizes. For instance, take a look at the microsoft.com site; squash the browser window and watch the content adapt to fit.
You might look at Zurb Foundation to get an idea of what can be done and the easy tools needed to get it working right away.
Google the terms to learn more.