Whatever the animal is comfortable with. He'll let you know when you've reached his limit :)
As high as the pony is comfortable with.
I would do about 2 ft 3 in. That is the hight that a pony that size would jump in a rated show. At the max I would jump 2'6
I would ask my vet because that depends on a huge number of things, breed, physical condition, rider size, conformation, etc. There's no one answer for all ponies that size and age.