Sperm competition is a key force driving evolutionary diversification of sperm morphology. In Caenorhabditis nematodes, sperm size is a major factor determining sperm competitiveness: male sperm is larger and consistently outcompetes hermaphrodite sperm, and larger male sperm outcompetes smaller male sperm. To generate an in-depth overview of sperm size variation in Caenorhabditis nematodes, we quantified male sperm size in 100 C. elegans wild isolates from a world-wide collection and in the 26 currently described Caenorhabditis species, including the three androdioecious species. Analysis of C. elegans wild isolates indicates significant levels of heritable variation in mean male sperm size, but Genome-Wide Association Mapping did not allow detection of any significant QTL underlying this variation. The observed sperm size distributions across strains indicate a complex genetic basis involving many loci of small effect. This survey of intraspecific variation also uncovered a strongly reduced mean male sperm size in LSJ1 and LSJ2 strains, recently derived from the reference strain N2. We show through quantitative complementation tests that a nurf-1 deletion in LSJ strains is responsible for reduced male sperm size, suggesting a role of the Nucleosome Remodeling Factor (NURF)-like complex in C. elegans spermatogenesis. Our quantification of sperm size evolution in different Caenorhabditis species confirmed earlier reports that mean sperm size of gonochoristic species is globally larger than in males of androdioecious species, although exceptions occur. While species of the elegans group all display relatively small, and similar, male sperm size, several gonochoristic species outside this group show convergent evolution of distinct, extremely large male sperm. In addition, within-species genetic variation in male sperm size is considerable in all examined gonochoristic and androdioecious species. We present experimental results aimed at understanding how germline and sperm development shape such dramatic variation in sperm size across species, isolate, sex and individual.