Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Size
SIZE_SIZE
Parameter: Variation
SIZE_VAR
Parameter: Over Age
SIZE_OVERAGE
Parameter: Age Gradient
SIZE_AGEGRAD
Parameter: On
IN_SIZE_ON
Parameter: Animation Time
IN_SIZE_ATIME