The content width in Chateau is percentage based. It's the overall width minus 21%. It starts with a #page-inner width of 1000px. There are two ways to widen it: make the whole thing wider or keep the overall width and shift the margins so the content is wider and the sidebar is smaller.
Here's an example for the first thing: making the whole thing wider and make the article area (including the date stuff on the left) 750px:
#page-inner, #footer-inner, #copyright-inner {
max-width: 1026px;
}
Here's an example that makes the .post-entry part (just the content column) 750px:
#page-inner, #footer-inner, #copyright-inner {
max-width: 1361px;
}