Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Width
PRIM_PLANE_WIDTHParameter: Height
PRIM_PLANE_HEIGHTParameter: Width Segments
PRIM_PLANE_SUBWParameter: Height Segments
PRIM_PLANE_SUBH