Many weight loss books or health related websites advocate the use of scales to make sure you keep to an accurate portion size when following a weight loss program. It is a good idea to do this, but would involve extra expense if you haven't already got kitchen scales.
An easier way is to get to know what a serving size looks like by buying a weight watchers meal. You will then see just what they suggest as a serving size. The plate doesn't look very big, but you know that it has been measured correctly for weight loss purposes.
Following the serving sizes on food labels can tell you how many calories there are per serve. Another good way to roughly judge portion size is, for example, if you are making chicken noodle soup from a packet, the nutrition label will state how many servings there is in one packet and how many calories per serve.
I personally don't weigh food, I simply judge the amount and roughly calculate the calories contained in my head. This way I feel confident that I stay to a good portion size to ensure weight loss.
If you do go and eat say McDonalds, or some other fast food type thing, make sure you order the smallest serving size, not the large size. Those fries will contain enough calories and fat on their own to ruin your daily allowance.