The moderators are supposed to use their own judgement when acting on an issue. The box is intentionally small so that the user can point to the issue and say, "please consider this for moderator action" - it is NOT meant to write an essay telling the moderator why it's wrong, or trying to convince them of the wrongness of the situation.
If there is an issue which requires a great deal of explanation, then you should either email the team@stackoverflow (yes, it does eventually get to the moderators) OR post something on Meta asking for moderator action (prior to the moderator alert these were the only two options available). Meta is also a good place to try to weigh in on why the situation should be moderated when you expect the moderator to waver on whether to act or not. I'd go so far as to say that you should only email the team in those cases where a long message is required and a certain amount of privacy seems necessary.
Please give us an example of a question which requires more than 150 characters to point out the problem. If this limit is a problem going forward, then it needs to be evaluated and corrected, and the best way to get to that point is by supplying examples.