changing the height of the submenus is not supported, nor is it strait forward.
you need to create custom css rules to target the level 1 and level 2 ULS:
#cssmw ul.level-1 li
#cssmw ul.level-2 li
to set the height.
you may also need to create other rules to target the a tags:
#cssmw ul.level-1 li a
to reset line hight and padding attributes.