Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Ellipse
PRIM_CIRCLE_ELLIPSE
Parameter: Ring
PRIM_CIRCLE_RING
Parameter: Radius
PRIM_CIRCLE_RADIUS
Parameter: Radius Y
PRIM_CIRCLE_RADIUSY
Parameter: Inner Radius
PRIM_CIRCLE_INNER