To seek unifying principles underlying adult body size in the phylum Nematoda, the volume of females of 3150 nematode species in 186 genera has been calculated from published length and diameter information; genera are grouped using the rDNA-based clades of De Ley et al. While the mass of females in some of the clades overlap, there are usually distinct differences within any particular clade between those nematodes that exist in living substrates and those that do not, the latter invariably being smaller. In all five clades (but not Chromadorida) the ability for females to achieve relatively great size is normal, but diminutive females are known from most clades and habitats. Bacterial feeding is common in females in non-living substrates and related females in living substrates, which may represent alternate generations, are often larger. If both groups of females are bacterial feeding it would help to understand the conundrum of whether those in living substrates are larger because they utilise better physical conditions or are larger because they are required to produce more propagules. Female body volume has previously been successfully used in zoogeography and further investigations may include not only adult volume but also address the question of stage-to-stage growth for which earlier studies revealed a paucity of data. This survey of the phylum has not been controlled for phylogeny, apart from the use of clades, and detailed studies could be made within clades. In particular, the effect of substrate, controlled through using species with alternate life cycles, should be tractable.