Yikes, it's a good that your car didn't break down in the middle of the desert; you could have been in big trouble. Also, I never realized that kangaroos were that indestructible!
He/She was full size, so about 2 metres tall and about 90kg. It was really trucking along, and the distance of each leap was 10 to 15 metres (based on its last bound and where it hit the car. I wasn't going to wander off into the desert to find it but it was nowhere in sight.