Twenty Eleven is a reactive design with a maximum width of 1000px. If you narrow your browser window below the 1000px, the theme will adjust for the smaller screen size and will work all the way down to the screen resolution of a smart phone.
[opinion]
On twenty eleven, given the sidebar, at 1000px wide, that gives about 550px for the content area, and that is a real good width for reading of text. Anything wider than that is actually more difficult to read (that’s why books aren’t 24″ wide).
Also, if you set the maximum width to something much wider, your header image will grow both in width and height and get seriously, seriously pixelated, not to mention that it would end up taking up the entire screen and there would be no evidence of anything but the header image.
[/opinion]
If you want it to be unlimited in width, then create a child theme (never modify the default theme directly as an unmodified version is sometimes needed to troubleshoot issues), and then add the following to the style.css file for the child theme.
#page {
width: 100%;
}
