You can use Reflection.
Spinner spinner = (Spinner) findViewById(R.id.spinner);
try {
Field popup = Spinner.class.getDeclaredField("mPopup");
popup.setAccessible(true);
// Get private mPopup member variable and try cast to ListPopupWindow
android.widget.ListPopupWindow popupWindow = (android.widget.ListPopupWindow) popup.get(spinner);
// Set popupWindow height to 500px
popupWindow.setHeight(500);
}
catch (NoClassDefFoundError | ClassCastException | NoSuchFieldException | IllegalAccessException e) {
// silently fail...
}