I need to make bigger hitbox. I can't hit my mob on the upper part of his legs.

I'm making a Titan mod so there are giant mobs in my mod. I increased entity model bounding box and game shows me the hitbox that I made but I can only hit below his legs. I saw other questions like this but there was some coding stuff that I didn't understand. How to open coding section? or Is there another way to fix that? (Sorry for my bad English)

This is how it looks (I didnt add texture yet so this was for test): https://imgur.com/Gd8967f