He isn't asking about spheres. He's asking about circumferences which are based on circles, and the equator is a circle.
Technically he's asking about the great circle on a spherical earth which corresponds to the equator. If the earth is not spherical then the calculation may be different.
The WGS84 datum is reference oblate spheroid used to define the earths surface. It was my understanding that using this datum the earth is defined to approximate a ruby ball, making calculation different. However having just checked it again I may well be wrong, in which case you can ignore everything I've written previously.
Also, since I'm going to be awkward, the answer is 1m (give or take). I got this by drawing a slice of pie and marking the two radii, the earths radius is so huge compared to the difference between the two radii that the end approximates a box 1m square. Hence 1m. I have no idea if this is a good answer.
Edited to add: Having thought about it a bit more, I think it's a rubbish answer. I'm going to give up now.