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For 236 of 650 Galactic open clusters identified in the ASCC-2.5 catalogue, we determine tidal radii from a three-parameter fit of King's profiles to the observed integrated density distribution of cluster members. The results are used to calibrate the observed sizes of the remaining clusters to a uniform scale of tidal radii of open clusters in the Solar neighbourhood. The tidal masses are computed from tidal radii. Within a distance of 850 pc where our sample is complete, the observed distributions of cluster masses can be explained by a general mass loss in open clusters with increasing age.