Stretch banner to fill fullscreen but keep the height minimum so on smaller screens, content inside won't overlap.

windowFullScreen.js

// jQuery Window FullScreen Plugin
//
// Author: Zeshan Ahmed
// Author URI: http://www.zeshanahmed.com/
// 
// = Replace .element_to_target with the your element selector

jQuery(document).ready(function($) {
  var $win        = $(window),
      $element    = $('.element_to_target'),
      $wpadminbar = $('#wpadminbar');

  function fullScreen( $el ) {
    // You can add addtional elements here to add offset value.
    // Mostly when fixed navbar is active and you want to offset
    // it's value from the fullscreen section's height.
    var winH        = $win.height(),
        wpadminbarH = $wpadminbar.height(),
        finalHeight = $winH - $wpadminbarH;
    
    // Replace "min-height" with "height" if you 
    // want to add fixed height.
    $el.css( 'min-height', finalHeight );
  }

  $win.ready( fullScreen( $element ) ).resize(function() {
    fullScreen( $element );
  });
});