I wouldn't leave the wet food out all day since it can go bad and make your chis sick. It's like leaving your cooked meat dishes out all day, unrefrigerated (not safe to eat).
In my opinion, Purina is not a good dog food. I would switch to a better dog food, such as, Royal Canin or Nutro Natural Choice since these foods are available in Ireland.
Bella, who weighs 5 pounds, eats 1/3 heaping cup of Merrick dry dog food (www.merrickpetcare.com
). This is divided into 2 or 3 servings each day (2 on weekdays and 3 on weekends). She gets 1,000 mg. fish oil and an enzyme supplement daily. We also give her small snacks of fruits and vegetables. She is a very active and athletic dog, and she burns off the calories.
In my opinion, the amount of dog food to feed a chi also depends on the individual chihuahua. Like people, one chihuahua's metabolism is different from another chihuahua.