Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Size
MDATA_SELECTLIVE_RADIUSParameter: Pressure Dependent
MDATA_SELECTLIVE_RADIUS_PEN_PRESSUREParameter: Visible Only
MDATA_SELECTLIVE_VISIBLEParameter: Tolerant Selection
MDATA_SELECTLIVE_TOLERANT_EDGE