Organ size is an important biological and agricultural trait. As the most vital photosynthetic organ and the primary source of human organic substances, intrinsic leaf size regulation has always been a hot field in biological research. Intrinsic leaf size is mainly determined by two closely related biological processes: cell proliferation and cell expansion. On the one hand, cell proliferation and cell expansion exhibit spatiotemporal differences. And on the other hand, both of them need precise regulation of primary cell wall. Here, we review the general process of plant organ growth and introduce the molecular regulation mechanism of intrinsic leaf size where we focus on the function of miRNAs. Not only some key genes of cell proliferation and cell expansion are regulated by miRNAs, but also some miRNAs can directly regulate cell wall components to regulate the final leaf size, indicating that the miRNAs are significant links in the genetic mechanism of controlling intrinsic leaf size.