Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Ellipse
PRIM_CIRCLE_ELLIPSEParameter: Ring
PRIM_CIRCLE_RINGParameter: Radius
PRIM_CIRCLE_RADIUSParameter: Radius Y
PRIM_CIRCLE_RADIUSYParameter: Inner Radius
PRIM_CIRCLE_INNER