I had a sleeve gastrectomy almost one year ago. No problems except for diarrhea sometimes for several days? Is this due to the stomach sized small.

Not really. Intermittent diarrhea could be irritable bowel, stress, or many other things. The sleeve does not cause malabsorption in general. Some studies have shown more rapid gastric emptying with a sleeve. Depending on ingested food, this could lead to more rapid flow of food through intestines (diarrhea). Might consider more fiber (fiber choice) or probiotics.
Think irritable bowe. Sleeve patients don't technically get dumping as they have an intact pyloric valve separating the stomach from the small bowel. This helps regulate the flow of food into the intestines. Dumping is more associated with gastric bypass where the valve is lost. Large quantities of high sugar drinks may empty quicker into intestines causing similar symptoms. May be irritable bowel or food sensitivity.