I would invest in a digital scale because it totally helped me learn my portion sizes (for everything from rice, veggies and meat. I tend to go by grams instead of ounces.)
I usually weigh raw and all before cooking or marinading so I know the calorie count beforehand. For now since you don't have a scale (yet), a serving size (110-120 calories) of chicken is approx. the size and thickness of a full deck of playing cards but I'm not sure if that comparison is for raw or cooked- anyone, please feel free to chime in on this.
I wouldn't worry too much about last night's chicken since what's done is done.
