Im not really sure what you mean by that. You can just adjust the values in the editor window you are in right now.
Its probably not something you can acess from within the game though, since the lightmap gets created during the light build process. You problably dont even need to acess them, just think of the value you enter in the Light map Resolution field as your “maximum” lightmap size. Mipmaps and lods will switch to a reduced version if you get too far away from your object.