I am barely 5' 1" and I don't think I could survive on 900 calories, either. I am not really strict about counting, but I would say I eat about 2,000 a day, maybe 1800 on days when I don't exercise. As I get older, it's definite balance of the kind of exercise I do, the intensity, and how many carbs I eat.
The only time in my life I had to lose weight (other than after my pregnancies) I lost 25 pounds by changing the way I ate (no junk food, not too much meat, lots of fruits and veggies) and starting to exercise. But, that exercise was mild compared to what I do now. I've never gone back to "bad" eating, but I go out a lot and I love to cook. For years, I controlled my weight by intense exercise, but that doesn't work so much now; I need to pay attention to the food part of the equation.