Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Size
FORMULAOBJECT_SIZEParameter: Effect
FORMULAOBJECT_EFFECTParameter: d(u,v,x,y,z,t)
FORMULAOBJECT_DLTParameter: X(x,y,z,t)
FORMULAOBJECT_FXParameter: Y(x,y,z,t)
FORMULAOBJECT_FYParameter: Z(x,y,z,t)
FORMULAOBJECT_FZParameter: Fit to Parent
FORMULAOBJECT_FITTOPARENT