That would be a matter of using CSS; ideally, you'd want to put it in one of your site-wide CSS files, so it'd be managed in just one place and affect all the SELECT menus you want affected, but you could even just put the CSS inline, like so:
<select name="o1" style="width:90%;max-width:500px;">
<option value="This">This</option>
<option value="That">That</option>
<option value="Other">Other</option>
</select>
Note I used both 'width' and 'max-width' in that order -- that would make the menu stretch to 90% of the width of its containing element, up to a maximum of 500px, beyond which it would not stretch any wider.
