how to change size of com.toedter.calendar.JDateChooser()'s arrow button

Question

I am use com.toedter.calendar.JDateChooser(). There are year picker and month picker in the GUI. There are two arrow buttons in each pickers. How do I change the size of these buttons? I cannot even get the button object because there is no public method to let me "getArrowButton()" so that I can setPreferenceSize().

To make it more clear, I want to change the buttons' sizes in red circle.