Anyone else reading.
Flat collisions should never be flat.
You need the collision to respond to things that could pass through it.
When dealing with walls which have a specific thickness you are usually bound by it.
When you are not - things like floor tiles for the ground level, or ceiling tiles for things which have nothing above - a 2m collision box is a good starting point to prevent just about anything from falling through the floor due to a physics misshap.