<div class="box">
<div class="box-container">
/* add this line */
<div class="row header">
<p>header (sized to content)</p>
</div>
<div class="row content">
<p>content (fills remaining space)</p>
</div>
<div class="row footer">
<p>footer (fixed height)</p>
</div>
</div>
/* end of box-container div */
</div>