Unless I'm missing something, pretty much every training routine I've ever seen on T-Nation tells exactly how many sets and reps to use. So, you'd use a weight that let's you perform those sets and reps, in that exercise.
For example, in the "Pillars of Strength" program you're doing now, Workout A has you doing the Full Back Squat, for 5 sets of 7,5,3,5,7. So, for the first set, you'd use a weight that let's you get 7 reps. Maybe you could do 8 or 9, but you'll only do 7. The next set, you'd go a little heavier, but only do 5 reps. Next set is still a little heavier, and you do 3 reps. And so on, and so on.
From set to set, you're going to use a weight that you can handle for the appropriate amount of reps. The weight I might use, or some other guy might use, is probably different than the weight you should use.
Everyone else is correct, though. If you're trying to get big, what you eat will be at least as important as what you lift. So, make sure you're eating enough calories each day, including a big breakfast, and a good post-workout shake, like Surge, followed by some good solid food.