How can you deduce the width, length, and height of an object using 2 planes in ARKit?

Question

I am trying to build a simple application with ARKit in Swift, my goal is to measure whether a cuboid object is larger than a certain dimension, to do this, I need to use ARKit to obtain the length, width, and height of the scanned object.

My idea is to use the ARWorldTrackingConfiguration() and its horizontal plane detection method to achieve this.

1. Classify a first anchor to the floor, and a second anchor to the top area of the cuboid.
2. Return the dimensions of the top area with planeAnchor.extent.
3. Return the perpendicular distance between the top plane and the floor.

I have attached a picture to better illustrate what I am trying to do, by doing this, I can obtain all 3 dimensions of the desired object.

I would like to get the width and length of m and the distance between m and n

I am currently facing 2 major roadblocks:

1. How can you classify/define two separate planes? One as the floor and the other as the bounded area on the top of the cuboid?

2. Are there any ways to return the perpendicular distance between two planes in ARKit?