Final answer:
Venus is the closest in size to Earth, making it Earth's "sister planet" due to its similar characteristics and proximity.
Step-by-step explanation:
The planet closest in size to Earth is Venus. Venus is often referred to as Earth's "sister planet" due to its similar size and proximity to Earth. Among the inner or terrestrial planets, which include Mercury, Venus, Earth, and Mars, Venus has a size most comparable to Earth.
Mars, although one of Earth's nearest neighbors in the solar system, is actually smaller than Venus and Earth. Unlike the Moon, which is Earth's satellite, and smaller dwarf planets like Pluto, Venus has a nearly circular orbit around the Sun and has geological features like mountains and volcanoes, indicating a more Earth-like composition and size.