Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Width
PRIM_RECTANGLE_WIDTHParameter: Height
PRIM_RECTANGLE_HEIGHTParameter: Rounding
PRIM_RECTANGLE_ROUNDINGParameter: Radius
PRIM_RECTANGLE_RADIUS