Mercury is significantly smaller than Earth. Here's a breakdown of the approximate size comparison:

* Diameter: Mercury's diameter is about 3,032 miles (4,880 kilometers), while Earth's diameter is about 7,926 miles (12,756 kilometers).

* Surface area: Mercury's surface area is about 28.9 million square miles (74.8 million square kilometers), compared to Earth's 196.9 million square miles (510.1 million square kilometers).

In simpler terms, Mercury is about 0.38 times the size of Earth in terms of diameter, or roughly 38% the size of Earth.