  We propose a proper method to measure the size of the narrow line region
(NLR) in distant quasars. The apparent angular size of the NLR is, in general,
too small to resolve technically. However, it is possible to map the NLR if
with gravitational lensing. In our method, we directly compare the observed
image of the NLR with the expected lensed images of the NLR for various source
sizes and lens models. Seeking the best fit image via the comparison
procedures, we can obtain the best-fit size and the best-fit lens model. We
apply this method to the two-dimensional spectroscopic data of a famous lensed
quasar, Q2237+0305. If the lens galaxy resembles the applied lens model, an
upper limit to the NLR size can be set 750 pc. Further, we examine how the
fitting results will be improved by future observations, taking into account
the realistic observational effects, such as seeing. Future observations will
provide us more stringent constraints on the size of the NLR and on the density
profile of the lens galaxy.
