Their are some problems with cuboid fields, can you change it back to radius, it is much easier to work with.

You can change anything cuboid to radius on the config file.

I just place a diamond block, then I typed /ps set radius 20, and it didn't work.

You are not forced to use cuboid. You have to use the field flag "cuboid" in order to enable them, If you don't use the field flag, then everything is the same as before. Not sure what you are requesting here.

P.S.
Cuboid problems should be fixed in 6.2.1

That's because the command is /ps setradius 20