That's really up to you. Figure out how much you need to keep you satisfied until your next meal or snack. I usually have a breakfast that's about 180 calories and that keeps me happy until lunch...well, I get hungry around 11:30 and then I just have lunch around 12. I think most servings of yogurt are around 6 oz. so maybe that's a good place to start.
First goal: under 180:
Second goal: 175 or below:
Third goal: 168 (no longer overweight):
Fourth goal: 160 or below:
Final goal: 145-155 (not sure if this will ever happen):