I am an exchange Code According to Your Instructions. The Result was that the height of the Box was reduced to 4 entries. I tried to change the 220px to 300px or 120px. This does not have to show any Effect.
When you view the code in the browser, there's this:
<select id="catid" name="catid" class="inputbox" size="6" multiple="">
The number "6" is in my opinion the Number of items displayed. When I change the Number "6" to the Number "15" appears to me all the Items in the list.
But I do not know where the PHP File I can not Number "6" to change.
Look at the Code in the image: