I do not think that reducing portion sizes is a viable option for most people. Unless you are already overeating - you will become hungry. You should switch to eating mostly fruits and vegetables like Danno says - they are much bulkier for the calories consumed. Limit the amount of rice you eat and reduce the meat you eat to almost nothing. Eliminate cheese and dairy.
Here is an excerpt from "Eat to Live" by Dr. Joel Fuhrman. Here he is talking about commercial diet plans like Jenny Craig that limit portion sizes - I think it is somewhat applicable to what you are talking about.
MOST WEIGHT LOSS PLANS ARE A WASTE OF YOUR MONEY
*What is wrong with every single commercial weight-loss program? THEY ARE ALL TOO HIGH IN FAT AND TOO LOW IN FIBER, because they cater to the American love affair with rich, high-fat foods.
*Weight Watchers' brand foods contain 24 percent of calories from fat. Lean Cuisine contains 25 percent of calories from fat. The Jenny Craig program requires the purchase of packaged meals with entrée's such as cheese soufflé and Salisbury steak, meals that are almost as bad as what most Americans eat at home. These commercial diet plans, since they are not very low in fat, must restrict portion sizes to offer 'low calorie' meals. These 'skimpy' portions represent an obsolete approach with a dismal tract record.
*It is merely a matter of time before those trying to keep their portions small increase the amount of food they are eating. The amount of fiber is insufficient, and the nutrient density of the diet is poor. These diets restrict calories, but because the food choices and meal plans are so calorie-dense, the dieters must eat tiny portions in order to lose weight.
*These choices don't satisfy our desire to eat, and we wind up craving food and becoming frustrated. When dieters can't stand eating thimble-size portions anymore and finally eat until satisfied, they put weight on with a vengeance. You may be able to hold your breath under water for a short period, but when you resurface you will be hungry for air and will be forced to speed up your respiratory rate. In a similar manner, if you cannot eat small portions forever, it just isn't likely to work for long.
*You can't eat out of boxes and consume powdered drinks forever, either. If you do lose some weight, you will always gain it back. Instead, permanent changes in your eating habits must be made. Learning new recipes and adopting different ways of eating that you can live with will maintain your weight loss and protect your health for the rest of your life.