How big is a 4 year olds bladder?

So I often wonder how big a 4 year old's bladder is.  You'd think it only holds a teaspoon or so of liquid if you are trying to drive somewhere (could be cross country or around the block) or sit through more than 20 minutes of a movie.  However if you ever have to clean up said contents of bladder, say from the floor or bedding, you will realize that a 4 year old's bladder can hold much...much more.  I am guessing that it must grow and shrink based on which size would currently irritate the mother most.  I can hear it thinking now "sweet we just got in the car, shrink!  shrink!"  or "hmmm tonights a good night to wet the bed, mom's gotta be somewhere early, lets expand this sucker!"  It is a truly amazing body part that from birth will continue to amaze from the first squirt in the face, to figuring out exactly how a gallon of liquid managed to soak your kids sheets that night.