system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do bodies know to "stop growing"? What tells the body to only grow your bones, organs etc to a certain size?
gpt
When the fetus grows, it turns on programs of genes, including triggers to stop. These programs can in turn trigger other programs until all the necessary programs have been executed. Some of these programs are kept open into our infancy, adolescence or even adulthood. This system of genetic control is fairly well characterized in Drosophila flys, c. Elegans flatworms and plants like arabidopsis, but much less understood in humans. What we do know is that the group of genes known as homeobox (Hox) play a critical role in triggering gene programs related to body architecture and therefore also height.

Interestingly, twin studies show a heritability of body size in the order of 60-70%, but attempts at pinpointing this to specific genes (in genome-wide association studies, GWAS) have largely failed. It is just, very, very complex.