Six heads may very well be the world wide average, I'm not quite sure. What I am sure of however is that in the video game industry six heads is too short. Video game design is to a large part ruled by current fashion, like it or not. And this is going to sound ridiculous but it's true anyway, being short is not fashionable, and have not been so for at least the last three hundred years. In the end it's your choice though, if you manage to pull it off it will look good no matter the proportions you chose.