It’s controlled by turning genes on and off, which causes certain cells to grow and differentiate, and triggers apoptosis ("programmed cell death") in others.
I found this. It’s pretty dense, but I think it answers your question: "The effect of insulin pathway activity on growth is largely autonomous to cells and multicellular regions, called compartments. Specific reduction of dAkt" (Drosophila Akt protein kinase) "function, an essential component of the insulin signaling pathway, in either the anterior or the posterior compartment of the wing imaginal disc results in a severe reduction of the respective compartment. … Recently, a novel signaling complex that restricts organ size by controlling both proliferation arrest and apoptosis has been discovered (Ryoo and Steller 2003). Mutations in either hippo, salvador, or warts result in a failure of cell cycle exit and in a protection from cell death, thus leading to massively overgrown organs. How an organ knows when it has reached its final size, however, is still mysterious and thus challenging."
That was more so about size of organs; this goes into patterns of growth: "An attractive hypothesis put forward based on a previous model of regeneration postulates that the individual cells of an organ primordium measure the concentration gradients of specific signaling molecules, such as Dpp in the Drosophila wing disc and Shh in the vertebrate limb bud (Day and Lawrence 2000). In immature small primordia, the gradients are steep and cells continue to grow and divide. Since the source of the gradient stays approximately constant, its concentration gradient flattens as the tissue grows. When the difference in the morphogen concentration sensed by the two ends of the cells along the axis of the gradient falls below a certain threshold, the cells stop growing."