How do organs and whole organisms know when to stop growing?
This is such a complex issue.
Embryology is a good introduction.
And the detail of the biochemical mechanisms involved in the major systems takes a lot of study to get the beginnings of an understanding.
And as with most things in the biology of life, the number of systems involved, and the levels of subtlety of interconnection and interaction is so great that we will probably be discovering subtle things about us should we live for the rest of eternity.
It is relatively easy to get the major systems described, just as it is relatively easy to map all the major highways in a country, and it is not so easy to know exactly where all the individual humans are, and what they are thinking, planning and doing – though mapping highways and traffic patterns will give you a good first order approximation of distributions. Same for us, particularly in the area of growth.
Many life forms (like most plants) don’t have a final size. Most trees just keep on growing each year. Some bristle cone pines are over 10,000 years old. Some creosote bushes appear to be over 30,000 years old. Some organisms get to a certain size and just seem to live on indefinitely (unless something kills them) – like the little freshwater hydra.
The chemistry and physics and logic involved is so amazingly beautiful, and it does take a few years of study to gain enough concepts and abstractions to start to see that beauty.