Stool problem: is it normal for your stools to the size and shape of a pencil one day and big the next day?

Yes. The size and shape of your stool is determined by how long the stool is in your rectum, and the tightness of your anal muscles. Long duration in the rectum = larger, harder stools. Tight anal muscles will make stool 'squeeze out' -- like out of a toothpaste container. You can stop these swings by getting more fiber (roughage) in your diet or taking benefiber; make sure to get enough water also!