Tip: The Best Set-Rep Scheme You've Never Tried

6 x 6 Using 70% of 1RM with Short Rest

This is one of the best set/rep schemes for size gains. In fact, it's one of the first. Legendary bodybuilding coach Vince Gironda called this loading scheme "the Mr. Olympia routine" because it was one he relied on when training Larry Scott, the very first Mr. Olympia.

It's based on a high training density (doing a lot of work in a shorter amount of time), not on load:

Perform 6 sets of 6 reps with a moderate weight that you could do for 10 reps. That's roughly 70% of your 1RM for the particular exercise you're doing. So it's not your 6-rep max.
Now it gets interesting... and challenging. You must complete all six sets in as little time as possible, resting only 30 seconds between sets. That's where the training density comes in.

While Gironda's recommendations were to shoot for rest periods of 30 seconds at most, he had his advanced bodybuilders resting only 15 seconds between sets.

Can't Do It?

Remember, the key factor with this loading scheme is density, not load. If you can't do all 6 sets with a strict 30 seconds rest, reduce the weight until you adapt to the short rest periods.