This dataset contains raw data of adult weight, growth rate, development time, ommatidium measurements and threshold weight of small copper used for analyses and figures in the research described in: Influence of photoperiod on thermal responses in body size, growth and development in Lycaena phlaeas (Lepidoptera: Lycaenidae). (2022) by Semsar-kazerouni, Siepel, and Verberk. In this paper, we investigated how temperature and photoperiod modify body size in the small copper, and whether size responses are related to responses in growth and development, threshold weight and cell size. Details about the experimental procedures are given in the Materials and Methods section of the paper. We have also included the R scripts for data analyses in this dataset. A brief description of the data files and column descriptions can be found below.