Change size of views exposed form search box
I think it would be better to change the form sooner than during themeing ... the best is to implement a form_alter hook, in this case


  function MODULENAME_form_views_exposed_form_alter(&amp;$form, $form_state)


you can try to dpm($form) first, to see what exactly you should change, and probably also add a condition about module id.

if you install advanced_help, this info can be found on your site at help/views/alter-exposed-filter :)