The casuarina moth, Lymantria xylina Swinhoe, is a serious defoliator of hardwood and fruit trees in Taiwan. Currently, overwintering egg mass density is the primary criterion used to monitor the population size of the casuarina moth. We assessed the relationship between casuarina moth egg mass size and the number of eggs per egg mass from two heavily infested sites in central Taiwan. Size parameters (length, width, area, and weight) were measured for each egg mass, and the egg number in each egg mass was counted. Results indicated substantial geographic variation in egg mass size. Regression analyses revealed a positive relationship between mass size (length, width, area, and weight) and the number of eggs per mass, and that weight explains ≈98% of the variation in the number of eggs per egg mass. The use of weight and other egg mass density measurements may provide forest and orchard managers with increased accuracy in estimating casuarina moth population size.