I think, you can use percentage width (instead of fixed width) for each of the div. For example: say there will be 4 buttons in a div. so, each of the div (which contains the button) width will be 25%.
<style type="text/css">
.div-parent
{
width:100%;
}
.div-button
{
float:left;
width:25%;
}
.div-button button
{
width: 100px;
}
</style>
The html will be:
<div class="div-parent">
<div class="div-button">
<input type="button" value="Button 1" />
</div>
<div class="div-button">
<input type="button" value="Button 2" />
</div>
<div class="div-button">
<input type="button" value="Button 3" />
</div>
<div class="div-button">
<input type="button" value="Button 4" />
</div>
</div>
Hope it helps