May I ask you how you solved the problem? I'm facing the same situation right now.
Via "scale" I set the desired scale for the NPC.
Via "height" I set the correct bounding box size for the NPC.
But the hit box is always messed up.
I added a picture for better imagination. It's alway the same, wether I scale the model to 115%, 150, 250 or anything else.