The planets in the Solar System that are most similar in size are Earth and Venus. Earth has a diameter of approximately 12,742 km, while Venus has a diameter of approximately 12,104 km. This means that Venus is only about 638 km smaller in diameter than Earth, making them the most similar in size among all the planets in the Solar System. Venus is also similar to Earth in terms of mass and density, making it the planet that is most similar to Earth overall.