I simply ask the question or say the starter provided, and hold my two hands close together to indicate starting small. Then, as he answers I move my hands a little further apart, as he adds more I move them further apart, if he adds some really great detail I swoop them out further apart, etc. His goal is to try for my hands to be totally far apart and extended like I am an umpire shouting 'safe' at a baseball game for a home run! 
I initially told him that his answers were itty-bitty (showing my hands close together), and that I needed his answers to be big and wide (showing my hands wide apart). This little tip has done more to improve his narration length and details in answers to questions than anything I've tried! Plus, he thinks it's fun. Just thought I'd share! In Christ,
Julie
