@julian thanks for the hint- yes i resize it show up. but i still notice some issue - after i click one category from the category picker window, the chose category didn't show in the parent category field. after i refresh the page, it then show up. but not a big issue.
i have a suggestion --- instead of using a popup menu window for a list of all categories, maybe just use a auto-complete text box to allow user to type in the category name.... this will be much simple when we have many categories (i.e more than 30....).