I'm a piscatarian (I eat fish.) maintaining lifetimer. Make wise choices and try to decide what you'll eat before you go somewhere. Then when your food arrives - child sized or adult - cut the plate into sections. For an adult meal, maybe into 3rds or 4ths, for a child's meal into halves. Eat that section, enjoy Disney a while, drink your drink, chat. Still hungry? Think you need more to fuel you for the next few hours? Size up your hunger and mentally decide how much you'll eat. Eat that portion. Stop. Enjoy Disney and your family. Still hungry? Eat some more. . .
You get the idea.
If you polish the food off, great, if not, great. But that way you don't over eat or eat mindlessly.
If I'm eating an adult portion at a restaurant, I nearly always look at the amount of food on my plate and divide it in half with my mind's eye. I eat my half by eating one quarter and then one quarter and usually at that point I don't want anymore. I'll leave the rest (if I'm traveling) or ask for a box (If I don't eat it, the dogs will)
And NO you do not have to finish every meal. It costs more, long-term to do so, in the cost to your health and happiness.