Possible evidence that the linear sizes of extragalactic radio sources were smaller at earlier epochs was first provided by the angular size redshift (θ-z) relation for double radio quasars (Legg 1970, Miley 1971, Wardle and Miley 1974). But because of the strong correlation between redshift (z) and radio luminosity (P) in flux limited radio samples, it is hard to decide if the observed decrease in sizes with z is caused by an epoch dependence of linear sizes (ℓ) or by an inverse correlation between P and ℓ. Several authors (eg. Stannard and Neal 1977, Wardle and Potash 1977, Hooley ET AL. 1978, Wills 1979, Masson 1980) have attempted to separate the two effects by comparing the properties of quasars from the 3CR survey with those from the 4C and Parkes samples. Although most of these studies appear to marginally favour a P-ℓ correlation, none of them can rule out even a fairly strong evolution in ℓ with z. Apart from the small numbers involved, the difficulty is that 3C and 4C quasars do not differ a great deal in their redshifts or luminosities. A complete sample of quasars at much weaker flux levels would be quite valuable in this regard.