How to show results in entityreference autocomplete only after the user enters 5 characters
One way to do it is create your own custom autocomplete (a good example is here) in your custom module. Then within your custom autocomplete function check if the submitted autocomplete string is 5-6 chars long then execute the search, like so (using the example from the tutorial above):

function demo_autocomplete($text) {
  if (strlen($text) &gt; 5) {
  // execute your code here:
    $results = array();
    ... etc
    drupal_json_output($results);
  }
}