Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
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