The "Sphere of Influence" mesh control is defined with a coordinate system, which has an origin at known x,y,z global coordinates, and a known radius.
Open the geometry editor and make a sphere of the known radius at the known global x,y,z coordinates of the coordinate system used in the mesh control and follow the rest of my post above to make a spherical region in the rectangular solid body. The spherical body can be assigned a different material than the rectangular body.
Then you can mesh the spherical body with the mesh "Body Size" control instead of the "Sphere of Influence" mesh control.
If you need help doing this, please create a Workbench Project Archive .wbpz file and attach it after you reply.