So, what would be the next steps to achieve it then?
How can I modify the bounding box, and/or the collision mask and the hitbox? -or is it easier creating a new block? ( I doubt it )
I need to clarify that I have never created a mod before, but I want to achieve this.
Well since you would actually need to create a mod for this, that's something you're going to have to ask elsewhere. This is the Resource Packs section. We don't know much about mods here... other than how to create resource packs for already existing ones that is. And even that's a bit of a mystery sometimes since modders make some very questionable design decisions at times.
Regardless, I suggest looking up modding tutorials and asking over in the Mods Development forum if you have any questions.