The iPhone XS has the same dimensions and same size display as the iPhone X. This means most iPhone X cases should fit the iPhone XS, but you should double check with your case's manufacturer before ordering any new device.
The iPhone XS camera bump is slightly larger than the iPhone X, which could prevent tight-fitting iPhone X cases from being used on the newer model. Click here for more details on the difference in size between the iPhone XS and iPhone X camera bumps.
The dimensions of the iPhone XS are 5.65 inches (143.6 mm) x 2.79 inches (70.9 mm) x 0.30 inch (7.7 mm). It weighs 6.24 ounces (177 grams) and has a 5.8‑inch display.