Compared to what? We can't give you a good answer because 3 kilometers is as big as 3 kilometers. If you'd said "How big is 3 kilometers in miles or in meters or compared to the Grand Canyon?" we could give you a good answer.
42.16 kilometers.
103000 square kilometers
Yes, 9 kilometers is pretty big compared to how fast a human can run.
140 kilometers is 86.99 miles.
177 square kilometers
5000 kilometers
Two kilometers is a distance equal to 1.24 miles.
The surface area is 750,000 square kilometers
Earth's surface area is 509,970,000 square kilometers.
17,098,242 square kilometers (excluding Crimea).
The area of Paris = 105.4 sq kilometers.
3 kilometers is 3,000 meters.