Well, it really depends from person to person. However, if you want a general guide, the Canadian Food Guide has a pretty good explanation of how much of an item is a serving.
For example
here you can find serving sizes for many fruits and vegetables. If you use the links you can find the same for the other categories.
To use that website, you need 20 grapes or 1/2 cup of watermelon.
I find, (based on that site) that you can usually expect 1/2 cup of a fruit or vegetable to work out to be a serving. There are a few that seem to require more/less. Like lettuce is more, and dried fruit is 1/4 cup.
Hope that helps.