Answer: Any positive number less than 16
The diagram shows AD = 16. Applying a dilation with positive scale factor less than 1, will make the figure shrink. Segment AD becomes smaller, so the new AD will be some length less than 16.
For instance, if you use a scale factor 1/2 = 0.5, then AD = 16 turns into A'D' = 16*0.5 = 8