The size of reptile hatchlings can be phenotypically plastic in response to incubation temperature, and size is a trait likely to influence fitness – i.e. hatchling size is proposed as an indicator of quality. The parental and incubation temperature effects on the size of one of New Zealand's most biologically significant reptile species, the tuatara Sphenodon punctatus are investigated. Artificial incubation at constant temperatures is used to produce founders for new captive and wild populations of tuatara and to augment existing rare populations. We compare size of hatchling tuatara from artificial and natural incubation treatments. The relationship of hatchling size with incubation temperature and sex is examined, and we investigate whether our results support differential fitness models for the evolution of temperature-dependent sex determination in tuatara. Initial egg mass is the most important factor affecting size of hatchling tuatara and is still an important influence at 10 months of age. Incubation temperature does not greatly influence size of hatchlings, but significantly influences size by 10 months of age. Constant artificial incubation conditions result in larger, but possibly less aggressive, juveniles than those from more variable natural incubation conditions by 10 months of age. Evidence from size patterns of tuatara incubated in natural nests supports differential fitness models for the adaptive significance of temperature-dependent sex determination. Thermal variation has little effect on size of male hatchlings, but female embryos that develop in more stable thermal conditions, in more reliable sites for hatching, are bigger and have longer jaws.