I don't believe it is ever specified what the diameter of the walls are. But によって their shape from what i can remember, they are not perfectly circular so a diameter wouldn't do much good for measurements. Now they might have specified the distance between the walls but i don't believe they did または i didn't see it.
In ep 1 it 発言しました that the distance between each is roughly the same. Maria to Rose - 100 km Rose to Sina - 130 km Sina to center - 250 km The diameter is 960 km So the ウォール is a bit bigger then France.
