The diagonal isn't representative due to the different aspect. In terms of area, they're just about the same (the iPhone 8 plus is actually just a very little bit larger). If you are viewing any 16:9 content on the X though, it works out to be an approximately 5" screen in effect.The 8+ has a 5.5 inch screen. X has a 5.8 inch screen. The X and XS have a larger screen with a smaller form factor.
https://www.gsmarena.com/apple_iphone_x-review-1681p3.php