If you want to keep the box heights equal and prevent overflow you could set the height and overflow properties in CSS for your box class:
.boxes
{
height:500px;
overflow: hidden;
text-overflow:ellipsis; // ends the text block with "..." - css3 no ie7/ie8 support
}
If you want the boxes to re-size dynamically based on the largest box you could use jQuery:
$(document).ready(function () {
var largestBox = Math.max($('#box1').height(),$('#box2').height(),$('#box3').height());
$('.boxes').height(largestBox );
});
Here's a jsfiddle demonstrating the jQuery option: http://jsfiddle.net/EjPSw/