If a recipe says it serves 5 then how do you measure it? If it is a casserole try to bake it in a dish that makes dividing the portions as easy as possible. If the recipe says 1 pound of chicken breasts and 3 breasts make a pound and it serves 5 it is more difficult to divide up. If you weighted the breasts before they went into the recipe and you cut them to divide into 5 servings that makes it a little easier. A meat loaf may serve 5 and that is pretty easy to divide but some times you just have to use your best judgment. Soup that serves 5 is tricky too, unless you put it in a quart measure to see how much you really have in the whole pot. If you have a ladle, fill a measuring cup with water and then pour into the ladle to see how much the ladle holds. One ladle I have holds 1/2 cup and another hold 3/4 cup. My W W class sells ladles and spoons that hold a measured amount of ingredients. That might help with the portions sizes.
Do the best you can if a ladle doesn't work and the item can't be weighed. Look in your W W book 1 Getting Started page 10. There are some guides on serving sizes. I know a deck of cards is surpose to be a portion of meat, poultry or fish.