If you look at the picture below you see a round planet based on a cube. surface mapped to it.
The idea is you select blocks on the same elevation level and doing so the system can generate a plane using the blocks surface area removing the round effect of the planet. Thus there is no need to have blocks that get larger toward their top in the build area.
Think of it as taking a bulldozer out to a site and leveling a huge field to build a large building. The difference here is the person simply uses a hand tool to select the build area.
So that the blocks fit and align they would need to use the size of the blocks on the surface areas face. That way they align to the ground and are comparable in size to one another. The other option is it creates a zone inside that area that uses 1:1 block sizes for building.
Either way it is an easy thing to do.