[en] The rapid fluctuations of radio and optical brightness which are observed for quasars, as well as for BL Lac objects, place severe upper limits on the possible sizes of these sources. These limits are not essentially changed by the assumption of a relativistically-expanding light source, contrary to widespread opinion. Thus the conflicts between fluctuation size limits for quasars and the minimum surface area needed to radiate sufficient power, if at cosmological distance, are not resolved by relativistic expansion