This probably cant be done. I will explain why.
The programing for the game. Follows certian perameters for the file it calls.
examle: this is the head file- /tophead//face/chin/sideheadright/side head left/back/ --- All thats cramed into one image the top left of that image might be top head and the parameters for that section of the might be 1,-2 -6y,-7x no matter what you change on the image the game is only going to call those perameters.
Now you could probably make the canvas of the head the same size as the canvas of the body that might make the head appear Larger but i doubt it will change the hit box perameters for it.
Hope i helped,
Debo