Dysfunction in adipocyte expansion during the onset of obesity is associated with metabolic abnormalities. Determination of adipocyte size and number is an important measure for a comprehensive evaluation of the metabolic status of adipose tissue. Here, we describe three methods for the determination of adipocyte size that can be applied to tissue samples obtained from humans and rodent models. While the first method presented is more robust, it does require the use of osmium, a toxic heavy metal, which requires special handling and disposal precautions in addition to specialized equipment. Two additional methods are described that can be of use to most researchers.