I agree with everyone else who said start with a small bag. Once you know it's what's right for her, go for the big one.

My aunt has a dog with an incredibly sensitive stomach from Irritable Bowel Disorder. She has to make certain nobody at the dog park gives him treats or he will be sick that night. She went through four different prescription diets before finding the one that works best for Levi, and she had a LOT of extra food that didn't work for him left over. She donated it to the shelter, but she could have saved a bit of money by starting with smaller bags.