Final answer:
In a scale model of the solar system where Earth is the size of a grape, the Sun would be approximately 1.4 meters in diameter, similar in size to an adult human.
Step-by-step explanation:
Imagine shrinking the solar system by a factor of one billion (109). In this scale model, if the Earth is reduced to the size of a grape, approximately 1.3 centimetres in diameter, we need to determine the corresponding size of the Sun. The true diameter of the Sun is about 1.4 million kilometres. Dividing this by our scale factor of a billion, we get approximately 1.4 meters, roughly the height of an adult. Therefore, on this scale, the Sun would be about the size of an adult human.