Congratulations on your new kitten! A feral kitten even smaller than that would not allow you to touch it, so it has been handled though it might not have spent time inside.

Kittens should be free fed until they are about 6 months old, and even up to a year is fine. They are like teenage humans, very fast metabolism and rapid growth, so they need plenty of good food. Use kitten food if you can, it is higher in fat.