2 or 3 lines are usually plenty. Long signatures are definitely frowned upon. Then you can correct whatever was wrong and post a new copy. Be aware, however, that some people may have already read the incorrect version so the sooner you cancel something, the better. Instead, if after careful editing, you have more to quote than to write, change the citation character.
Most newsreaders have two ways to post a news article.
This is used whenever you are starting a new topic. It is important that these two methods not be confused. Conversely, don't use the followup mechanism to post an article that is an unrelated thread. Violating this convention sometimes leads to confusion and annoyance of users with threaded newsreaders. In preparing an article, be aware that other people's machines are not the same as yours.