How big can a Hydra get?

I'm curious, just how large can a Hydra grow to be? Is there a specific size limit, or does it depend on various factors such as its environment, diet, and age? Are there any recorded instances of exceptionally large Hydras, and if so, what were their estimated dimensions? Additionally, how does the size of a Hydra compare to other creatures in its ecosystem, and does its size play a significant role in its survival and reproduction?