DisplayObjectContainer size

Hi, i'd like to know if it's a bug, or if this works as it was supposed to: (this was tested in html5 only)

for example, if you create a TextField with a text content, and add it in a new sprite instance with the "addChild" method and then if you want to know the size of the whole stuff by retreiving the sprite's dimension, you get
width = 0, height = 0,
so it seems that the DisplayObjectContainer doesn't care about its children to return its size.

I guess if it's not a bug that i'll have to write some handler which updates at the Event.ADDED event and reset some "visibleSize" property by looping above all the children to see which one stands the most on the left, top, and finishes the most on the right, bottom.