In order to do the trifold, you roll or press into a rectangle. How big should this rectangle be?

I don't think you need to aim for a specific size for the rectangle. Even though you aren't doing all of the same folds, the step-by-step photos in this puff pastry post might be of assistance: https://food52.com/blog/13101-the-cheater-s-pain-free-puff-pastry