Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Width
PRIM_POLY_WIDTHParameter: Height
PRIM_POLY_HEIGHTParameter: Segments
PRIM_POLY_SUBParameter: Triangle
PRIM_POLY_TRIANG