Final answer:
Uranus is the planet with a diameter larger than that of Mars but smaller than that of Neptune.
Step-by-step explanation:
The planet that has a diameter larger than Mars but smaller than Neptune is Uranus. According to the provided information, Mars' diameter is about half of Earth's, Neptune is one of the gas giants along with Jupiter and Saturn, and Uranus is between the size of Saturn and Neptune. The descriptive term mini-Neptune refers to planets that are between the size of Earth and Neptune, but since Uranus is a known planet in our solar system and fits the size criteria between Mars and Neptune, Uranus is the correct answer.