How big a portion do you need ?
What you need depends on how big you are anyway; an 8 stone lady will want to eat less than a 16 stone man. The only portion sizing that is common today is adult/child. So if they reduce portions are not larger people (who might well not be overweight) go hungry?
So will we see small/medium/large portions in restaurants ? If we do I would expect them to be priced differently and unfairly since the cost of the chow raw ingredients is generally a small part of the cost of a meal.