Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Width
PRIM_RECTANGLE_WIDTH
Parameter: Height
PRIM_RECTANGLE_HEIGHT
Parameter: Rounding
PRIM_RECTANGLE_ROUNDING
Parameter: Radius
PRIM_RECTANGLE_RADIUS