The problem is, that changing one width might not be enough, because there can be a wrapper-div with a limiting width for all the content, or you may have to make a sidebar smaller, etc pp...
In addition, a lot of people (and themes) use different div-IDs for the content-divs in different templates... for example
<div id="narrow"> in index.php, and
<div id="wide"> in page.php, and so on. I would recommend that you backup your css and then just try changing some widths. It might also be a good idea to read up on some basic css knowledge, for you to get an unterstanding of what you're doing.
If you need help, you should provide a link to your blog, otherwise it's almost impossible to assist you in any way with specific issues.