use one of the free tools from the web (for instance firebug for firefox) to investigate which style is esponsible for the post width.
or look into the htmlm code of your post (or front page) in the browser (‘view’ ‘view source’ in the browser tool bar) and look through the code, looking for the html tags and css classes or ids, before your post title or post content starts.
possibly something like (or similar to):
<div id="post-<php...?> class="post">
or post a link to your site, and someone might be able to point the right style out to you.
be aware that a change to the post’s width might not be enough, as there might be other restrictive styles around it.
