Line 11 adds the event listener to the BorderContainer (line 41). The Transform event is passed to the onZoom function on line 14, which has a member variable of type Sprite. By reading the scale of the event's scaleY property, you can enforce a minimum size as shown by the if-else statement on lines 17-26 and assign the new values. Note that the minimum scale size this can go to is 1. This is to prevent users from scaling the Sprite too small to place both fingers on it to zoom again.
The code may be downloaded from here. You will need Flash Builder 4 and AIR 2.5 or later to run this.