Yes, it’s a confusing aspect ration, but used by many of the social media channels (FB, Insta, Twitter). Here are some example sizes that all have 1.91:1 aspect ratio.
Note: Aspect ratios are Width x Height. If you divide each of these examples Width/Height, you will get 1.91 (For example 1080/566 = 1.91)
800 x 418 pixels
1080 pixels x 566 pixels (Size I use for Instagram horizontal)
1800 x 945 pixels
Hope that helps.