I've been caught by this before. It only happened once, and it will only happen once to you too, as from this point forward you know that comments have a limit, and if you accidentally try to submit a comment that is too short, you know what the message means.
I'm a little on the fence on this feature request, because it's honestly not hard to update the text. It's just text. But the more stuff we keep adding to the UI that is inconsequential just serves to decrease the signal to noise ratio, which isn't good.
The worst thing that would happen to you and I in this case, had we not realized "oh, the message is talking about length", is that we would just get frustrated and not post our not-valuable comment. There would be no big loss to the community. Eventually, we'd catch on, which we both did. ;)
I'd prefer to focus on improvements to the UI that encourage people to improve closed questions instead of angrily running away from the Q&A model; I'd prefer to focus on UI changes that support quality Q&A. Adding more text in hopes that it may help a subset of users understand something that isn't critical to Q&A just serves to drown it out. Speaking from experience, I've seen UI's get so cluttered to the point where people are using bold red to drown out red text and blink tags with 18pt font to help make it stand out over the bold red. ;)
One of Stack Exchange's strengths is in keeping a clean UI, so I would hope that there would be more data of people sitting on a comment screen for minutes on end clicking submit and scratching their head, wondering why it doesn't work, before impulsively making this change to the text.
In short, I'm not suggesting it's a bad idea, but I am suggesting that there should be data used to determine if this is truly necessary.
I like @animuson's suggestion to change the word "more" to "characters". This is sort of like changing a generic pronoun to a concrete noun, and doesn't increase the total word count of the message.
2 characters to go...