Using simultaneous observations of the MDI and EIT instruments on board the SoHO spacecraft, we determined in flight the plate scale of the EIT. We found a value of 2.629+-0.001 arc seconds per pixel, in fair agreement with the 2.627+-0.001 arc seconds per pixel value deduced from recent laboratory measurements of the focal length, and much higher by 7 sigma than the 2.622 arc seconds per pixel value of the pre-flight calibrations. The plate scale is found to be constant across the field of view, confirming the negligible distortion level predicted by the theoretical models of the EIT. Furthermore, the 2 sigma difference between our results and the latest laboratory measurements, although statistically small, may confirm a recent work suggesting that the solar photospheric radius may be 0.5 Mm lower than the classically adopted value of 695.99 Mm.