@sebastián-cisneros In static:api.routes, you'll want to call controllerHelpers.setupAPIRoute to mount the route to the appropriate /api/v3/plugins mount point.
You can see an example of how we do it in the quickstart plugin
Now there is only max length for title but I think it should be also for posts.
Actually, IMO, it's not just a feature request, it could be even a security problem to allow a user to post a message with hundreds of thousands of characters and then try to parse it with each plugin (that's what happened to me).
@xuso thumb up!