We propose a proper method of measuring the size of the narrow-line regions (NLRs) 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 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 of 750?pc. Furthermore, we examine how the fitting results will be improved by future observations, taking into account 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.
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