Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Type
GUIDEOBJECT_TYPE
Parameter: Line Mode
GUIDEOBJECT_LINE_MODE
Parameter: Spatial Mode
GUIDEOBJECT_SPATIAL_MODE
Parameter: Size X
GUIDEOBJECT_SIZE_X
Parameter: Size Z
GUIDEOBJECT_SIZE_Z
Parameter: Axis Centered
GUIDEOBJECT_PLANE_CENTERAXIS