Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Bounce
DEFLECTOROBJECT_ELASTICITY
Parameter: Split Beam
DEFLECTOROBJECT_SPLIT
Parameter: Size X
DEFLECTOROBJECT_SIZEX
Parameter: Size Y
DEFLECTOROBJECT_SIZEY