Custom width/depth on entity hitboxes

So I noticed that width and depth are set to the same variable, which makes sense enough for bipeds and a few other models- but I have a rather large entity that isn't square shaped and the square hitbox actually makes its behavior wonky and the front clips through walls. I'd like to know how I can set a depth variable separate from the width variable, and I'm not afraid to get my hands on coding a little. Just need someone to point me in the right direction. TIA