cup (not the "mega-mugs" so common now) or other similar dish as a bowl. I have an old set of dishes where the coffee cups are 8 ounces. It's easier to estimate the appropriate portion size with smaller dishes. You can also use other non-traditional dishes/containers that hold less than today's larger plates/bowls/mugs. Goodwill and antique stores may have smaller dishes, just be sure that anything you use is LABELED SAFE FOR FOOD, so that you aren't exposed to unsafe coatings or dyes.
Not an earth-shaking suggestion, but having a "special" container helps make smaller portions look better and helps train the eye to recognize what appropriate portion sizes look like.