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!