Like AnthonyWJones said, this seems like a CSS/HTML question. Once you use XSLT to transform the RSS feed into HTML, it is simply treated (and thus rendered) as a standard HTML document.
My best guess at this point is that you need to put a container (a block level element such as <DIV>) around the description (or the whole document) and specify a width for it that is within the dimensions of the display using CSS.
So something like this should work:
<style type="text/css">
#container {
width: 90%;
}
</style>
<div id="container">
// Your XSL code
</div>