get familiar with one of the free browser tools, such as firebug or firefox web developer add-on.
these should make it much easier to locate css ids and classes for the various elements on a page.
http://www.jesnyder.com/4-2/
I did use the google developer tool and it said the div is #primary and i tired that but it didnt seem to work
On the home and other pages (and maybe single posts, I don’t know), the rule that’s governing the width is:
.one-column #page {
max-width: 690px;
}
On a blog type page, the width is set by this:
#page {
margin: 2em auto;
max-width: 1000px;
}
So there’s where you need to look.
I think you need to learn a bit how to use the Google Chrome tool. You have to move up the html tree to find the element highest up the tree that’s setting a width.
OK, it’s a bit more complicated than that, but as a rule of thumb that’s what you need to do.
Cheers
PAE