I definitely feel your pain here. In Ghost 1.0 we extended out the hard limits across many fields like bio, tag description etc. We kept the soft-limits in place in Ghost itself whilst we figured out exactly what a more sensible limit looks like for these fields and rework the UI to support more data.
Currently there’s no easy way to adjust these limits but if you’re on Ghost 1.0 you can either modify the code, and build your own version Ghost or put extra data directly in the database. The latter is much easier, but may result in validation errors when trying to use the UI.
Soft limits live in Ghost Admin and the server-side Schema.
Out of curiosity, what’s the use case here? What are you planning to do with the data in the field and how many characters do you think you’ll be looking to store? Real world use cases help to inform us on how these fields should work in future