That too is easily modified. Mastodon hardcodes the value a few places, or at least used to. Pleroma has config settings to separately enforce a local limit on how much you're allowed to write, and a hard limit above which it will drop remote posts. Problem is the latter - any instances which increases the post length too much will need to consider that they'll effectively make federation unreliable for their users because most other instances will have a hard limit above which they'll drop or truncate posts.

But overall it's easy to iterate on this, and you can try things on your own instance or your own "corner" of likeminded instances while still be able to interoperate on most things.