When it comes to things like meats, fish, oils, and other liquids, serving sizes are often given in measurements such as cups and ounces, which may be hard to visualize. Some items around your home can be helpful when it comes to knowing how big (or small) a serving really is.
One serving of meat is about 3 ounces. This is about the size of a deck of cards or a bar of soap.
A serving of cooked fish is about 3 ounces as well, which is almost equivalent to the size of a checkbook.
When considering one serving of grains, you can think of a piece of bread, or even one pancake about the size of a disc.
For more general guidelines, you can think of a cup as the size of a baseball, a tablespoon as the size of a poker chip, and one serving of oil (1 teaspoon) as about the size of the tip of your thumb.