We know almost nothing about the evolutionary tracks of extragalactic radio sources but those tracks are, however, strongly constrained by the distribution of sources in the radio luminosity, P, overall physical size, D, diagram. This is the radioastronomer's H-R diagram, an analogy which two lines of algebra shows is exact. Fig. 1 is the P-D diagram for the 3CR 166 source sample of Jenkins et al. (1977) with later additions. Most of the sources are identified and have known redshifts. It is a flux density limited sample so that the numbers at any P are weighted relative to the true space density by P3/2 because of the differing volumes of space sampled. The important feature of the diagram is the lack of sources greater than 1 Mpc in size. Because of doubts about the completeness of the sample in this region, we have made searches in the 6C 151MHz survey for sources having surface brightnesses lying between the two lines of slope 2 on the right of Fig.1. The numbers found to a limiting flux density of 1–2 Jy suggest that there is no serious underestimate of the numbers in the 166 source sample.