You can also use the CSS "width" property in the above code.
Firebug is an excellent tool which will help you to see how the code works and test CSS changes in the browser before editing site files.
I also recommend using
the Codecademy course on HTML & CSS to lean the basics; it's free, and it will save you a lot of time to not have to wait for a response on the forum when it's something basic that can be done in less than a minute on your own.