Alrighty, so let's get started. According to this, we know that the world of Witcher is around 136 square km, compared to Skyrim's 36 square km. That is a huge distance within itself and it is also a huge size. Within the video I posted, you saw that whilst moving in a skell, you move much faster than how you can walk in both Skyrim and in Xenoblade itself. Of course, this will need a little bit of testing, but a light framed Skell on the ground could easily move 10 times faster than you can walk.
I say this because a typical skell is around 5 times the size as a human and that moves proportionally as well. Then the car properties of a Skell doubles it's movement speed. So that leaves Xenoblade X to be around 10 times that of Skyrim, at 360km(with some ground to spare of course). So yeah, those are the proportions you are looking for. Plus, not only that, but as we have seen, Xenoblade X's world is heavily layered with caves, areas that are up high, plateaued, ect. So that even pulls more credence to this fact.