Little is known about how mammalian cells maintain cell size homeostasis. We conducted a novel genetic screen to identify cell-size-controlling genes and isolated Largen, the product of a gene (PRR16) that increased cell size upon overexpression in human cells. In vitro evidence indicated that Largen preferentially stimulates the translation of specific subsets of mRNAs, including those encoding proteins affecting mitochondrial functions. The involvement of Largen in mitochondrial respiration was consistent with the increased mitochondrial mass and greater ATP production in Largen-overexpressing cells. Furthermore, Largen overexpression led to increased cell size in vivo, as revealed by analyses of conditional Largen transgenic mice. Our results establish Largen as an important link between mRNA translation, mitochondrial functions, and the control of mammalian cell size. Display omitted •PRR16/Largen is identified in a screen for genes controlling mammalian cell size•Largen enhances mRNA translation•Largen increases mitochondrial mass and activity•Largen controls cell size in vivo Using a genetic screen, Yamamoto et al. identify PRR16 as a controller of mammalian cell size. Overexpression of the PRR16 gene product “Largen” increased cell size by modulating mitochondrial activity and mRNA translation in an mTOR-independent manner, both in vitro and in vivo.