Step-by-step After Effects tutorial using Expressions.
This tutorial shows you how to use a Javascript expression, linked to a slider, to set the width of a text layer. This keeps the scale correct, but let's you control how many words per line you want and makes it easy to adjust.
The expression appears complicated, but I take you through it line by line, explaining what each element does. But if you're not interested in code, which is totally fine, then you can download the preset from the link below.
[Edit: Yeah, I know I've changed the name, but my wife hated it and the joke was starting to get old. Plus this brings these videos inline with my other two series, Makin' and Startin']