Yes, you can either edit the XML and reupload so that nothing has to be reverted later if changers are to be made, or edit the template.
Either way, in the template in your styles (or in the XML) find this:
Code:
<input type="text" name="q" size="21" maxlength="255" value=""></input>
and change the size="21" to whatever shorter size fits. 21 is what I use on my site. In the future I may make that an adminCP variable.