Addendums should never be more than a page.
To avoid giving the wrong impression, you should always highlight the positive, rather than the negative. For example, if the addendum refers to bad grades, emphasize the improvement in your grades over time.
It is only worth it to submit an addendum to explain truly bad grades, otherwise unexplained absences from school or to submit an addendum regarding "why I want to go to X law school" if one school is truly your top choice.
Although many people do submit addendums, it is always a personal decision whether it will actually help your application. The best advice is to concentrate on making the personal statement/overall application package the best it can be so that there is no need to submit additional essays. Good luck!
Alisa Greenstein | Veritas Prep Admissions Consultant | www.VeritasPrep.com/law