Body size is one of the most important axes to understand a large biodiversity. An amazing diversity in body mass of lives ranges over about 21 orders of magnitude, from a tiny bacteria such as Mycoplasma weighing 10-13g to a giant Sequoia tree weighing 109g. As a consequence of this variation, nearly all the structures and functions of organisms are constrained with body size, from the molecular, cellular and whole-organism levels to the ecological and evolutionary dynamics. These relationships are well described by the allometric equation. In this note, we introduce backgrounds to focus on some important correlates and consequences of body size, in particular on energy metabolism at the level of individual organism. Metabolism of an individual organism reflects the energy and material transformations that are used for both the maintenance of existing structure and the production of new biomass. Although body size is a primary determinant for metabolic rates, metabolism-body size relationships, in particular within species, i.e., the ontogenetic changes of metabolism with growth have not been well established in many species. The metabolic scaling in biology still keeps an intriguing and enduring problems.