Beam size is being measured successfully in the EXT line of ATF2 by means of a multi-OTR (Optical Transition Radiation) system made of 4 OTRS. In the talk the design, construction and installation issues will be summarized. Furthermore an analytical work about the reconstruction of the emittance by using the beam size measurements will be also shown. From these studies we could obtain general guidelines about the location of the measurement stations to avoid unphysical emittance results.