Some CSS help

Yo /prog/
I've got some basic CSS problem that I'd love help with
At the moment I've got one div for a main area that I'd like the rest of my content in, and then have 3 divs for my content (left navigation, middle content, right information, all floating left). However, I have trouble configuring my main area (I want to enclose everything in a border). I can set the width because it's static, but I the height is dynamic depending on the amount of content. However, I can't find a way to do this correctly.
If I don't fill in the height property, the main area div has a height of 0px since it has no content in it. If I set the height property to 100%, the height of the main area is always the size of your browser or something so if you have too little or too much content it's ass.
I've probably made some design mistake (unless I'm missing some obvious CSS property), how would I go about unfucking this?

>>5
That doesn't work, because if there is too much content the main area won't stretch all the way to the bottom

Not going to pick up a book on CSS just for this, I've got plenty of programming experience and just can't fix this