Bats (Order Chiroptera) often are photographed by automatic cameras with infrared sensors set in forests for wildlife surveys. To facilitate species identification of the photographed bats, we have developed a technique to estimate their approximate size using their shadows. This technique gives unbiased size estimates, but its precision depends on the accuracy of parameters used (camera-position parameters and on-film measurements). Used with due care, this technique could be a useful tool to identify some species of automatically photographed bats. Because this technique is in spreadsheet format, the estimates and their confidence limits are obtained simply by filling in the required parameter values and their possible level of errors.