It’s hard to tell without seeing one, but I am betting it’s identical and already purchased an iPhone X case from last years model and plan to use it for my new XS model. I hope I’m right.
They look very similar, but in checking the accessories forum, people are reporting that there’s a slight difference in the size of the camera bump (supposedly, it’s slightly bigger in the Xs). Don’t know how much the difference is or if that’s accurate, but I’d guess as long as you buy/have a case for the X that’s not super-tight to the camera, you’ll be fine.
I’m sure size wise they’ll be fine, but depending on the case, the bottom may need to be adjusted by the manufacturer as it’s been changed compared to the original X.
Ah, yes, I see. Most cases I've used have a large cutout top and bottom which clears all ports, speakers and mics. I suppose those are the ones which fit X and Xs.
Having said that, if you have a precision case it should still fit, you'd just have a few cutouts where they are not necessary - the Xs has fewer holes than the X, certainly at the bottom. In the worst case scenario an X case would be OK until you buy something more specific.