How can I make a mob with sword bigger?

Hi, I'm new to modding. I want to have in my mod a tall monster that uses a sword through this option:

• Equipment: Allows a Biped/Zombie modelled mob to have the equipment. From left to right the slots represent the right hand, then the left, then the head, chest plate, leggings, and boots.

I have already tried using a big custom model but the sword part doesn't appear due to that type of model isn't supported for that. Here is my first question, is it possible to make my custom model available to this option?

I've also tried using the pre-set model "biped" and it worked for the sword, but there's not an option to increase the height, so it doesn't help me either. Is there something I'm missing, maybe something with code?

Thanks :)