Say : "Could I have half a serving?" ( but not all places in America will allow that) Often people will order a children's meal, which contain smaller portions than a regular meal.

Hello, I would keep it as simple as possible to limit interpretation and avoid confusion. So something measurable is perfect. So if you know exactly how much you want, say so: half a portion, two-thirds of a portion - then you will get exactly what you ask for. But honestly, two-thirds of a portion will sound pretty strange. Otherwise, simply say: "May I please have a slightly smaller portion than what is normally served?" However in this instance this can lead to interpretation: what is small to you could be even smaller for someone else - so you may not be entirely happy with the amount served. Hope this helps! If you wish to learn more about specific ways of expressing proportions and measures, feel free to visit my teaching page and get in touch! Micha.