The general rule of thumb is that 15g of Carbohydrates represents about 1 diabetic serving. Depending on your diet, a dietitian can evaluate the daily amount of carbs you need in a given day.
There is great information on this website that outlines specific serving sizes for each food group. This can give you information to understand portion size.
Here are some charts for fruits: http://myplate.gov/printpages/MyPlateFoodGroups/Fruits/food-groups.fruits-amount.pdf and veggies: http://myplate.gov/printpages/MyPlateFoodGroups/Vegetables/food-groups.vegetables-amount.pdf