How big is a HashKey?

Excuse me, I was just reading about cryptography and the use of HashKeys in digital security, and I couldn't help but wonder - how big is a HashKey exactly? Is it a fixed size for all applications, or does it vary depending on the specific cryptographic algorithm or purpose it's being used for? And if it does vary, what are some common sizes or ranges that I might encounter in practice? I'm just trying to get a better understanding of the underlying mechanics and how they're applied in the real world.