The Cocker Spaniel diet will need to include animal proteins and carbohydrates, vitamins and minerals, and omega fatty acids--nutrients these dogs require to maintain their health in the long term. This means the best Cocker Spaniel dog food is premium dry kibble, as it has balanced portions of the above-listed ingredients. If fed a high-quality food like Royal Canin, Cocker Spaniel dogs will be much healthier than if they subsist on cheap, generic food--which has mostly empty "filler" ingredients that simply don't meet a dog's nutritional needs.
Royal Canin is a brand that has a line of "breed-specific" food formulated specifically for the Cocker Spaniel. Royal Canin Cocker Spaniel Puppy Food, Royal Canin Cocker Spaniel Junior, and Royal Canin Cocker Spaniel Adult dog food are each formulated for this breed at specific ages.
Specifically how much should a Cocker Spaniel eat? Adults will need about two cups of premium food per day, divided into two meals. Portions of food for Cocker Spaniel puppies are a bit smaller: depending on a pup's age, about 1½ cups per day, divided into three meals (not two) until six months old.
For more info, here's a handy Cocker Spaniel puppy feeding guide:
Dog AgeDog WeightFood TypeAmountFrequency2 Months4 lbsDry (Puppy formula)0.25 cups3x/day3 Months8 lbsDry0.4 cups3x/day6 Months15 lbsDry0.5 cups3x/day9 Months20 lbsDry* (Puppy/Adult)0.8 cups2x/day11 Months+25 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. These dogs will easily become overweight if constantly overfed--and a fat Cocker Spaniel will have major health problems. You can help control your Cocker's weight by having consistent feeding and exercise schedules and by not leaving food in the Cocker Spaniel food bowl all the time.
If you're worried your Cocker is overweight, try running a hand along the dog's side; if you can't feel any ribs, it's diet time--which means less food and more exercise!