iPhone XS: 13.4 cm x 6.2 cm
iPhone XR: 14.1 cm x 6.5 cm
iPhone XS Max: 15.0 cm x 6.9 cm
iPhone 8 Plus: 12.2 cm x 6.8 cm
For the 8 Plus (and older Plus models), the entire screen is used for 16:9 video. For the XR, XS, and XS Max, there will be black bars on the sides in landscape mode, which means the limiting dimension is the height of the video, which on phones means the width.
As you can see, the 8 Plus has a screen width of about 68 mm, whereas the XS Max has a screen width of about 69 mm, nearly the same.
This page explains the issue in terms of last year's iPhone X. Below are pictures of the 8 vs X vs 8 Plus. In the first picture, the entirety of the video is shown on all phones. In the second picture, the video on the X is zoomed to fill the screen, cutting off the top and bottom of the video.
iPhone XR: 14.1 cm x 6.5 cm
iPhone XS Max: 15.0 cm x 6.9 cm
iPhone 8 Plus: 12.2 cm x 6.8 cm
For the 8 Plus (and older Plus models), the entire screen is used for 16:9 video. For the XR, XS, and XS Max, there will be black bars on the sides in landscape mode, which means the limiting dimension is the height of the video, which on phones means the width.
As you can see, the 8 Plus has a screen width of about 68 mm, whereas the XS Max has a screen width of about 69 mm, nearly the same.
This page explains the issue in terms of last year's iPhone X. Below are pictures of the 8 vs X vs 8 Plus. In the first picture, the entirety of the video is shown on all phones. In the second picture, the video on the X is zoomed to fill the screen, cutting off the top and bottom of the video.