At the very beginning, you'll probably be able to lose just by writing down everything you eat, without regard to quantifying it.
When that stops working, something like Fitday makes it easy to estimate
calories, as long as you can estimate quantity / weight / volume.
When I was first losing, and was doing it by portion control rather than calorie counting, the right portion size was "less than I'd have eaten before." There is no one right answer for everyone. If you're counting
calories, the right portion size is "one that lets me stay within my calorie range," but again, there's no one right answer for everyone.
For the stir-fry, if I were counting
calories (so needed to be able to enter quantities into Fitday), I'd eyeball the chicken and guesstimate ounces or cups, then do the same for the peppers and the rice, and add some oil for it being stirfried.
I don't obsess over getting my calorie counts right if I eat out, unless I have nutritional information handy. I do try to record everything with my best guess, but sometimes a guess is the best you can do.
