Leaf size influences plant development and biomass and is also an important agricultural trait in Brassica crops, in which leaves are the main organ produced for consumption. Leaf size is determined by the coordinated regulation of cell proliferation and cell expansion during leaf development, and these processes are strictly controlled by various integrated signals from the intrinsic regulatory network and the growth environment. Understanding the molecular mechanism of leaf size control is a prerequisite for molecular breeding for crop improvement purposes. Although research on leaf size control is just beginning in Brassica, recent studies have identified several genes and QTLs that are important in leaf size regulation. These genes have been proposed to influence leaf growth through different pathways and mechanisms, including phytohormone biosynthesis and signaling, transcription regulation, small RNAs, and others. In this review, we summarize the current findings regarding the genetic regulators of leaf size in Brassica and discuss future prospects for this research.