In a Nature Comm. paper published in July 2019 the Bosch lab shows that size in Hydra is controlled by Wnt- and TGF-β signaling. Insulin-like peptide receptor (INSR) and forkhead box protein O (FoxO) are important genetic drivers of size determination controlling the same developmental regulators.

Learn more about this research in a (German) radio interview: 

https://www.mdr.de/wissen/mensch-alltag/warum-menschen-unterschiedlich-gross-100.html