The vast majority of desktop web users will view a site in a window that is maximized. See these articles from 2006 (before mobile browsing took off):

http://mentalized.net/journal/2006/10/24/browser_size_does_m... (this one is the most interesting because its data is collected from a variety of sites)

http://www.baekdal.com/insights/actual-browser-sizes

And this one from 2012:

http://beantin.se/post/32275497175/computer-screens-browser-...