You could create a new skin, and make it whatever height you wish.
can u explain me? how to skin it?
I use FB3, and to do this I use Illustrator CS3. Details are here:
http://www.adobe.com/devnet/flex/articles/skins_styles.html?utm_source=feedburner&utm_medi um=feed&utm_campaign=Feed%3A+developer_center_flex_tutorials+%28Adobe+Flex+Developer+Cente r%3A+Recent+tutorials%29
Thanks for your feedbacks.
Finally I found the one solution for set the thumb height,
var sBar:*=comboBox.dropdown.getChildAt(3); //Get the combo box drop down's scrollbar instance
var scrollThumb:ScrollThumb = sBar.getChildAt(2) as ScrollThumb; //Get the scrollthumb instance of Scrollbar instance
scrollThumb.minHeight = SOME HEIGHT;//Finally set the minimum height for scroll thumb.