I remember consulting with YodaJammies over the dimensions when I was rendering and he suggested around 17" as it is a machine pistol rather than a sub-machine gun.
I may have supplied you with an older version prior to the scaling change, I apologize if this causes any set back for you.
I suppose it all depends on the fit of the grip to your fingers.
Do you have any images of it being held by an adult?
It's hard to gauge the size with your nephew.