Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Grid Size
FFDOBJECT_SIZEParameter: Grid Points X
FFDOBJECT_XSUBParameter: Grid Points Y
FFDOBJECT_YSUBParameter: Grid Points Z
FFDOBJECT_ZSUBParameter: Fit to Parent
FFDOBJECT_FITTOPARENTParameter: Reset
FFDOBJECT_RESET