YOU WALK INTO the restaurant, resolute. You are going to eat a light, healthy, low-calorie salad and nothing else. You try to ignore the sights and smells of the hearty dishes around you. You order the salad and your friends order the salad plus a main dish. They will be served a substantial meal, you an empty plate.

However, the salad bar has much to offer. You start with a little of this, a little of that. Before you know it, you are running out of space on your plate. You crowd it in. Your salad is getting embarrassingly large but it's too late to put anything back. You need two large scoops of dressing to cover it. You walk slowly back to your seat, balancing 10 inches on your plate.

And you eat it all, but for some reason all those raw vegetables don't fill you up like a plate of pasta. But Even so, if you had an automatic calorie counter attached to your belt, you might be shocked. A large salad with loads of dressing could very well put on more pounds than that plate of pasta.

It's obvious from the above accompanying chart that the calories from salad don't come from raw vegetables, which are practically calorie-free. What contribute to a fattening salad are generous portions of dressing and side-dish-type side offerings like potato and macaroni salad. Add several types of cheeses, and a few different toppings like croutons and imitation bacon bits, and the calorie count can go out of sight.

The best way to keep a salad from turning into the calorie-equivalent of a banana split is to keep the portions small and to be selective. Don't try everything--even if it will it just won't fit on your plate. Stick to the raw vegetables and skip the side dishes. If you can manage to come back to your table with a salad that doesn't embarrass you because of its size, the calorie total probably won't upset you too much either. Go easy on the dressing, or use a low-calorie one if it's offered. Getting the Most Nutrients

In general, salads offer a well-balanced, high-quality, vitamin- and mineral-rich meal, according to Ruth Matthews, a nutritionist with the U.S. Department of Agriculture's Human Nutrition Information Service. Here are a few pointers for ensuring the widest selection of nutrients:

* Choose dark green leafy vegetables like spinach and romaine lettuce over iceberg lettuce, if available at the salad bar, since the dark green vegetables are higher in vitamin A and folic acid. content

* A wide selection of vegetables will usually provide a nice balance of vitamins, says Matthews. Tomatoes provide vitamin C, broccoli both vitamin A and C. Bean sprouts add a significant amount of minerals.

* You can easily meet your protein needs at a well-stocked salad bar. Legumes like chick peas provide not only protein but are a good source of potassium, Matthews notes. Calcium can be obtained from the cheeses, vitamin B-12 from the eggs. Sesame seeds and imitation bacon bits, which are made of soy, can also contribute to protein needs, says Matthews.

* Dressings have little to offer nutrient-wise few nutrients to offer except for Vitamin E, says John Weihrauch, also of USDA's Human Nutrition Information Service. Contrary to popular belief, commercially prepared creamy and oil-based dressings don't vary much in calorie content (see chart). Weihrauch advises use of low-calorie dressings, if offered. If not, he says, use sparing amounts dressings sparingly.