In 1926, Haldane published an essay titled On Being the Right Size in which he argued that the structure, function, and behavior of an organism are strongly conditioned by the physical forces that exert the greatest impact at the scale at which it exists. This chapter puts Haldane’s insight to work in the context of contemporary cell and molecular biology. Owing to their minuscule size, cells and molecules are subject to very different forces than macroscopic organisms. In a sense, macroscopic and microscopic entities inhabit different “worlds”: the former is ruled by gravity and inertia, whereas the latter is governed by Brownian motion. One implication is that we should be extremely skeptical of models and analogies that seek to explain properties of microscopic entities by appealing to properties of macroscopic ones. Unfortunately, this is precisely what the appeal to engineering metaphors in molecular biology attempts to do. Molecular biologists routinely resort to such metaphors because they are familiar and intuitively intelligible. But if our machines were the size of molecules it would be impossible for them to function the way they do. It follows that we should avoid distorting biological reality by construing it in engineering terms. This chapter examines four key metaphors of molecular biology – “genetic program,” “cellular circuitry,” “molecular machine,” and “molecular motor” – and argues that their deficiencies derive from their neglect of scale. It also tries to explain why many biologists today appear to have forgotten the importance of scale that Haldane drew attention to in his essay. It is suggested that the reason has to do with the influence of Schrödinger’s argument in What is Life? regarding the stability of the gene.