Need help with understanding bounding boxes

Hello. I am having trouble trying to figure out bounding boxes.

The values and parameters are all very confusing for me and I don't know how to use them - when I try and fiddle around with them either the bounding box doesn't look how I want it to or it completely dissapears, making the block indestructable and impossible to interact with.

For reference, I am trying to figure out how to make a bounding box similar to the 4 stages of amethyst crystals: Small Amethyst Bud, Medium Amethyst Bud, Large Amethyst Bud and Amethyst cluster.

I would appreciate if someone knew how to make sense of all these parameters and such.