Good question!
As for the first oneList in the itemgallery (card)images and large images, they all use the exact same aspect ratio of 1.91 (width) to 1 (height). The Messenger mobile app, Facebook chat box, and Messenger browser version are all the same.
The recommended size is 909×476 px, which makes your media look flawless on almost all devices.
This is the default for this size:
If you don't have an image at that resolution, most users probably won't notice a drop in quality if the image is at least 500×262 pixels.
This is the default for this size:
Images sent using “image” have a dynamic aspect ratio.
The baseline is that the closer the image is to a perfect square, the better. This way, it looks larger and more prominent.
It's perfectly fine if the image is a bit longer on the width or height, but wider than taller.
The recommended size is 900×900 pixels - using this resolution, your images will be larger and clearer.
This is the default for this size:
If you don't have an image at that resolution, an image of at least 500×500 px will still work.
Please note that sending a 900×500 image is not a problem. Just don't make one side longer than twice the other side.
Additionally, please note that images sent using “image” display differently on different devices. Here are some details that may not be very interesting and may change frequently.
When displayed on Messenger, the aspect ratio of the image can vary between 1:2 and 2:1 (width: height). Any image taller than 1:2 will be cropped at the top and bottom, so users will only see the middle part. Therefore, any image wider than 2:1 will be cropped on the left and right sides, showing only the middle part.
When viewed on the desktop version of Messenger in a browser, wide images are displayed as wide as the user's screen allows, and if too large, they are scaled down proportionally to maintain their aspect ratio.
If the height of a tall image is more than 2.2 times its width, the tall image will be cropped at the top and bottom.
In Facebook's chat window, the display ratio of images is almost the same as the original version. Overly long or tall images will appear small, so it is strongly recommended not to use them.
*Please remember that FB documentation states that images over 10Mb cannot be displayed in Messenger, although in most cases they may be uploaded to the flow.*
