Maviskw, what I've gathered when they talk about "an onion" or what ever that may be, that's about a tennis ball sized amount. From being on a diabetes diet that is also the size of what "an apple" or "an orange" is. Most store apples are at least two servings. From the control diet you want the "lunch box" sized bag of small fruits. A banana is two servings, again, visualize that tennis ball size.
You can also learn it by holding one (a tennis ball that is). For me, it's about as much as I can "claw hand" out of something chopped, or just under a cup -- but you can use one cup as a standard. But then it depends on how strong that onion is or what the flavors your family likes.