HTML

<div class="outer">
    <div class="inner">

        <!-- your content -->

    </div><!-- end .inner -->
</div><!-- end .outer -->

CSS

.outer,.inner {
    display: block;
}

.outer {
    /* specify fixed width */
    width: 300px;
    padding: 0;
}

.inner {
    /* specify padding,can be changed while remaining fixed width of .outer */
    padding: 5px;
}