Text box length for search in header

I need help changing the size of the form field in the search function in the header of my site.
Here is the link: http://96.0.126.204/store/
I have changed the width of the search box in the left column just fine by modifying tpl_search.php. However, similar attempts to widen the field in tpl_search_header.php have been unsuccessful. Is this the correct file to modify? (if so, maxlength is set to 30, and it should be wider based on that number)

I am using my own template based on mostlygrey. I have also added editable sideboxes. I have looked in the forum for answers, but couldn't get something drilled down to this one minor detail, especially with the changes that I have already attempted.

Thanks in advance,
Laura