An empirical investigation of elementary school teacher candidates on classification activities dealing with animate and inanimate objects in terms of being living or non-living demonstrates that as the size of the objects increases, subjects are more likely to classify them correctly as either being an animal or having living characteristics. Despite a variety of misconceptions having an impact on the results, size magnitude is shown to play a significant role on proper classification. The subjects' performance on these activities at an advanced stage of their preparation suggests that their factual and procedural knowledge are deficient due to a lack of opportunities for conceptual development of the items tested. The identification of the role of size on the proper classification of objects in the activities bears significantly on the science curricular structure at the elementary school level. As the results of this study indicate, both pre-service elementary school teachers and by extension their prospective students need longer practice dealing with living and non-living classification activities, particularly in tasks where the microscopic features of matter can be investigated so that the proclivity to regard size as the defining characteristic is effectively addressed.