Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
GUIDEOBJECT_TYPEParameter: Line Mode
GUIDEOBJECT_LINE_MODEParameter: Spatial Mode
GUIDEOBJECT_SPATIAL_MODEParameter: Size X
GUIDEOBJECT_SIZE_XParameter: Size Z
GUIDEOBJECT_SIZE_ZParameter: Axis Centered
GUIDEOBJECT_PLANE_CENTERAXIS