-Short replies don’t help — you need to write like you're helping someone.
I disagree on this, short replies are always direct and straight to to the point and most forum members prefers short posts, they often carry more quality. Most lengthy post usually discouraging to read through and most times they deviate from the point they are trying to make making it difficult for the audience to understand.
It's true that posts that are too long can discourage many members and prevent them from reading them, but it is also true that shortest ones can go unnoticed, especially if written by newbies.
There is a healthy in-between, not too short nor too long, but in the end everything has more to do with the quality of your ideas rather than with the length of your writings.