Mars-sized.
Being water-rich or not is irrelevant, according to the Theia-impact theory.
Regards,
While Theia may explain the origin of the moon, where did all the water come from? If we assume that the pre-impact Earth had similar amounts of water (percentage-wise) as Venus & Mars, must we assume that the percentage of water on the smaller body was the same or greater than Earth’s? If greater, then it is most likely the smaller body was not from our system. If the percentage is similar, that does not explain why Earth has the high percentage of water that we do which is far more than twice the percentage of our planetary neighbors.
Interesting, is it not?