I often look at the amount of protein in a recipe to guestimate how many servings it should be (or whether a serving will be filling enough for me). For a recipe like this, with lots of other ingredients, I would expect 4 oz of meat to be plenty for a single serving. You've got 2 lbs of turkey going into it, so right there the recipe should easily be eight servings, which would be about 400 calories a serving, completely doable on most diets. On top of that, you've got four (count 'em, thats four!!) cans of beans going into it. All those beans have protein too, so, with all of them in the casserole, I seriously doubt you'd need 4 oz of turkey per serving to feel full and satisfied. My guess is that eight servings is actually too few. You could probably divide the soup into ten or twelve servings, which would make the calories per serving even lower.
You haven't listed how many grams of protein are in the total recipe, but that can also help determine the serving size. I generally like my dinner to have 20 to 25g of protein. With all those beans, this is going to be a really high protein dinner, so I'd probably assume 25g of protein per serving. Take the total protein, divide by 25 and that might be another way of figuring out how many servings it should be. Then you can divide the number of servings into the total calories to figure out the calories per serving.
My Before and After Pics
"Decide what you want; decide what you're willing to exchange for it; establish your priorities, and go to work." --H.L. Hunt
"Life has hills. Set the treadmill at an incline."