SelectList: content frame size

Hello guys, how are you? Im new to Vaadin, and Im trying to set the size of the content frame in a SelectList to the same size of the SelectList caption, but I havent discovered a way. I want the size to be equal when the list is empty or not empty, because the SelectList is dynamically refreshed. Im attaching two pics that will give better explanation of what I want. Thank you for your time!