<p>Our knowledge of how and why various biological and ecological traits relate (scale) to body size has grown at a rapid pace. In this exploratory analysis, I aim to further advance our knowledge of biological scaling by arguing that individuality and selfhood increase along a size spectrum from tiny microbes to huge trees and whales, as driven by a gradient of decreasing mortality. According to a proposed size–self spectrum (SSS) model, tiny short-lived organisms are continuously engaged in relatively rapid, all-consuming, selfless reproduction, whereas large long-lived organisms have evolved relatively high levels of individuality and self-preservation. Grades of individuality in organisms along the SSS are recognized by their levels of development of (a) protective boundaries between their inner and outer environments, (b) autonomy and identity preservation, and (c) self-awareness and regulation. Paradoxically, as increasingly large organisms have evolved increased independence from their external environments, they have also exerted greater per-capita impacts on them. With increasing body size, the prevailing direction of influence of an individual organism’s internal versus external environments switches from inward in small ‘exocentric’ species to outward in large ‘endocentric’ species. Implications of the SSS model for understanding the nature and environmental impact of humans, who are relatively large organisms, as well as various other controversial biological, ecological, and philosophical issues are discussed.</p>