The size of Theia is a subject of ongoing research and debate, with no definitive answer. However, scientists have developed models and theories based on the available evidence. Here's what we know:

* Similar size to Mars: The prevailing theory suggests that Theia was roughly the size of Mars, with a diameter of around 6,779 kilometers (4,217 miles).

* Similar mass to Earth's moon: This theory is based on the composition of the moon, which is thought to have originated from the debris of the Theia impact.

* Range of possibilities: Some scientists believe Theia might have been slightly smaller or larger than Mars. The exact size and mass of Theia are still subject to ongoing research and new discoveries.

Important Note: These estimates are based on models and are not definitively proven. We are still learning about the early solar system and the formation of the Earth-Moon system.