If you don't care about a minimum width for the div and really just don't want the div to expand across the whole container, you can float it left -- floated divs by default expand to support their contents, like so:
<form>
<div style="float: left; background-color: blue">
<input type="button" name="blah" value="lots and lots of characters"/>
<input type="button" name="blah2" value="some characters"/>
</div>
</form>