  Energetic gas outflows driven by active galactic nuclei (AGNs) are considered
as one of the mechanisms, by which supermassive black holes affect their host
galaxies. To probe the impact of AGN-driven outflows, it is essential to
quantify the size of the region under the influence of outflows. In the third
of a series of papers, we present the spatially-resolved kinematics of ionized
gas for 3 additional type 2 AGNs based on the Gemini Multi-Object Spectrograph
(GMOS) integral field spectroscopy. Along with the 6 AGNs presented in our
previous works and the 14 AGNs with available GMOS IFU data, we construct a
sample of 23 luminous type 2 AGNs at z < 0.2, and kinematically measure the
size of ionized gas outflows by tracing the radial decrease of the velocity
dispersion of the [O iii] {\lambda}5007 emission line. The
kinematically-measured outflow size ranges from 0.60 to ~7.45 kpc, depending on
AGN luminosity. We find that the size of the photoionized region is larger than
the kinematically-measured outflow size, while the flux-weighted
photoionization size is significantly smaller. Thus, using the photoionization
size as a proxy for the outflow size will lead to overestimation or
underestimation, and introduce a large uncertainty of the mass outflow rate and
the energy output rate. We report the outflow size-luminosity relation with a
slope of 0.28{\pm}0.03, which is shallower than the slope of the correlation
between the photoionization size and luminosity.
