There's supposed to be a more user friendly error message when the length is too long. Have you already done any customization? Is this a clean Flarum install, or which extensions are you using? Can you share your php flarum info output?
I believe there are a few discussions about this topic on the forum already. It's basically down to the database column length and the Laravel validator rule for the post content. And checking all third-party extensions are compatible with this change, which they usually should be.
