Is it normal for one testicle to be slightly larger and hang lower than the other one?
Yes, the two testicles should hang at slightly different heights. See the answer to the previous question for details.
It is also normal for the testicles to be slightly different in size, especially while you are growing. Growth in humans is rarely smooth or even. While various parts grow approximately in concert, some will get ahead or lag behind the other. Even after your growth in completed, it is not unusual for one testicle to end up larger than the other. If everyone took exact measurements, you would find that most have slight variations between the left and right side -- not just in your testicles, but also in your hands, feet, arms, and legs.
What would not be normal is for hard lumps to appear on the surface of your testicles. These lumps could be signs of cancer and should be seen by a doctor immediately.