Generally, the limit shouldn't be an issue, it's a default - the forum software (Discourse) runs on a trust system, but as you haven't contributed to other topics yet, it considers you a "basic member":
In this instance, I've overrode your level to "member" so the limit has gone away for you. If you wanted to learn more:
So now you can fire away!
Discourse has done pretty good to catch spammers and their dirty tricks, but it could catch legitimate users or cases like this with a lengthy topic.