Every time I have made split pea soup, I have to try to estimate the amounts to be finished in one meal. If I store the leftover soup (in the refrigerator or the freezer), the soup thickens to the point that it's barely liquid anymore.   In Ham and Pea Soup with Whole Dried Peas, there's a passing comment that split peas break apart more to thicken soup. Is there any way of reducing this effect or simply predicting the amount of thickening that will happen? No matter how much liquid I add to the original soup, I can't seem to store the leftovers without getting a porridge-like texture.  
This is not a defect but a characteristic of pea-soup. In Holland, the pea-soup is considered good when a wooden spoon can be put upright in the pot. I've never eating it that thick myself, though.  The starches and the gelatin will form a mesh when cooling, so the consistency of the pea-soup will always be thick when cold. On heating, do what Mien advised: add some water or broth to the soup to the consistency that you want.  You will have to heat slowly because otherwise the soup will stick to the bottom of the pan and burn.  Don't forget to thoroughly heat the soup and let is simmer for at least 10' to kill any unwanted visitors (germs) before serving. 