Body size is a crucial metric that is often obtained during the breeding migration of female land crabs. Most commonly, these measurements are obtained by hand but hand-measuring them carries risks for both crabs and researchers. In this study, we developed a novel “photo-image approach” to measure the maximum carapace width of Cardisoma carnifex (Herbst, 1796) in the field, and compared the results with hand measurements. We found a significant relationship in the maximum carapace width measured between the two approaches (Linear regression, , ). The difference (0.54 ± 0.18 mm) and relative error (0.61 ± 0.19%) in the maximum carapace width measured between the two approaches were minimal. We suggest that the photo-image approach is feasible for determining crab size in the field. The approach holds promise for its application in citizen science projects, facilitating advancements in crab research.