The size of something depends on what exactly you are referring to. For example:

• Earth, as a planet, has an equatorial diameter of about 12,756 kilometers (7,926 miles) and a surface area of around 510 million square kilometers (197 million square miles). It is the fifth largest planet in the solar system with a mass of about 5.97 × 10^24 kilograms.

• The United States of America covers about 9.8 million square kilometers (3.8 million square miles), making it the third-largest country by land area in the world.

• A human stomach, by contrast, is about 12 inches long and 6 inches wide when empty, and can expand to hold about 1 quart of food.

To provide an accurate answer, the specific object or entity you want to know the size of is needed. Please specify what "how big is" refers to.