It's really quite simple:
HTML:
<div>
<section class="left">
</section>
<section class="right">
</section>
<div class="clear"></div>
</div>
CSS:
div, section { border: 1px solid #000; }
.left { height: 100%; width: 200px; float: left; height: 200px; }
.right { width: 300px; float: right; height: 300px; }
.clear { clear: both; }
Fiddler: http://jsfiddle.net/H2c6g/
How can I make the div occupy the full height?