Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Size
PRIM_CUBE_LEN
Parameter: Segments X
PRIM_CUBE_SUBX
Parameter: Segments Y
PRIM_CUBE_SUBY
Parameter: Segments Z
PRIM_CUBE_SUBZ
Parameter: Separate Surfaces
PRIM_CUBE_SEP
Parameter: Fillet
PRIM_CUBE_DOFILLET
Parameter: Fillet Radius
PRIM_CUBE_FRAD
Parameter: Fillet Subdivision
PRIM_CUBE_SUBF