Use 2 different font sizes on combobox form elements
If it's possible in HTML &amp; CSS flavour you are using, it is. There are two ways:


Purely CSS one, described at w3schools and not really belonging to this site as it's Drupal independent.
Class way - to get more flexibility and compatibility, add classes to your &lt;option&gt; tags inside your &lt;select&gt;. You can do this implementing theme_select() in your theme. You would need to replace form_select_options() with your own function providing similar functionality, or parse original one's output to add your classes.