How do I adjust autocomplete list width
As coded in autocomplete.js file inside misc folder, popup is wrapped inside 

&lt;div id="autocomplete"&gt;&lt;/div&gt;


You can able to set width using above id. If you want to disable hide popup option for time being to style the popup results, then try commenting lines shown below in autocomplete.js file inside misc folder of your drupal root.

/**
 * Hides the autocomplete suggestions.
 */
Drupal.jsAC.prototype.hidePopup = function (keycode) {
  // Select item if the right key or mousebutton was pressed.
  if (this.selected &amp;&amp; ((keycode &amp;&amp; keycode != 46 &amp;&amp; keycode != 8 &amp;&amp; keycode != 27) || !keycode)) {
    this.input.value = $(this.selected).data('autocompleteValue');
  }
  // Hide popup.
  var popup = this.popup;
  if (popup) {
    this.popup = null;
    $(popup).fadeOut('fast', function () { $(popup).remove(); });
  }
  this.selected = false;
  $(this.ariaLive).empty();
};


NOTE: Remove commented code once styling is done. Clear the cache after making javascript changes and make sure changes are reflected in autocomplete.js file using firebug. Clear browser cache.

Hope it helps :-)