My suggestion: Keep it succinct, manageable and strong.
You haven't said why you're writing an addendum, but the fact that you are means that there's something in your application that requires clarification, or more explanation. In other words, a negative that you'd like to turn into a positive.
So personally, I'd say that you shouldn't be giving your readers the burden of a lengthy paper to read - added length may also signal defensiveness.
I suggest filling 2/3 of one page with 1.5-spaced text. That will guarantee that it'll be to the point, and also will be read.