Player Hitboxes

So I want to be able to edit the player's hitbox. For example, if you press f3+b while playing minecraft, you'll notice you can see your hitbox. Then, when you start swimming, the hitbox shrinks down to be one block in size. Now I want to do that, specifically I want to be able to change the player's hitbox to be one block in size. Is there a way to do this? 

Thanks in advance