Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Size
PRIM_CUBE_LENParameter: Segments X
PRIM_CUBE_SUBXParameter: Segments Y
PRIM_CUBE_SUBYParameter: Segments Z
PRIM_CUBE_SUBZParameter: Separate Surfaces
PRIM_CUBE_SEPParameter: Fillet
PRIM_CUBE_DOFILLETParameter: Fillet Radius
PRIM_CUBE_FRADParameter: Fillet Subdivision
PRIM_CUBE_SUBF