The ECS diet will need to include animal proteins and carbohydrates, vitamins and minerals, and omega fatty acids--nutrients every dog requires to maintain its health. The best English Cocker Spaniel food is premium dry kibble, as it has balanced portions of the above-listed ingredients, and is much better for preventing obesity (a common issue with these dogs). Cheap, generic food is not recommended for this breed because it contains mostly empty "filler" ingredients that are harder to digest and cause significant weight gain.
Recommended brands include Royal Canin, Blue Buffalo, and Diamond, which have excellent lines of premium food.
They typical adult ECS will need about two cups of premium dry food per day, divided into two meals. ECS puppies, depending on age, will need a bit less: about 1½ cups per day, divided into three meals (not two) until six months old.
For more info, see this English Cocker Spaniel feeding chart:
Dog AgeDog WeightFood TypeAmountFrequency2 Months5 lbsDry (Puppy formula)0.25 cups3x/day3 Months10 lbsDry0.33 cups3x/day6 Months20 lbsDry0.5 cups3x/day9 Months25 lbsDry* (Puppy/Adult)0.85 cups2x/day12 Months+30 lbsDry (Adult formula)1 cup2x/day*--Around this time, transition to adult food by first mixing in a bit of adult formula with the puppy formula. Over the course of a week, with each meal add a little more adult food to the mixture, until the dog is eating it entirely.
Try if possible to stick to the above-listed portions. If constantly overfed, these dogs will quickly become overweight--and a fat English Cocker Spaniel will have numerous health problems. Help control your dog's weight by having regular feeding schedules and by not leaving food in the dog's bowl all the time.
If you're concerned your ECS is overweight, reduce its food consumption--and increase its daily exercise.