Nope, I would never tell you to choose a different theme … well, maybe I would, only if it were in your absolute best interest.
If the theme has a custom CSS area, add the following to it and if it does not, add it to the end of its styles.css file:
width: 120px !important;
That should resize the input width. It will also affect the search width above the template too. I can’t remember if I had a link to your site or not, sorry, But I could give more specific CSS so only the widget is affected if you could share a link to your site.
Hope that helps.