Good question!
As for the Gallery image and the Card image, they both use exactly the same aspect ratio, which is 1.91 (width) to 1 (height). It’s the same across Messenger mobile app, Facebook chat box and Messenger browser version.
The recommended size is 909x476 px, which makes your media appear flawless on almost any device.
Here is the preset for this size:
If you don't have images of this resolution, most contacts won't probably notice the quality drop at all, if the image is at least 500x262 px.
Here is the preset for this size:
Images that you send with Image Block have dynamic aspect ratio.
The baseline is that the closer your image is to a perfect square, the better. This way it appears larger and more prominent.
If an image is a bit on the wide or on the tall side, it is totally fine, but wide is better than tall.
The recommended size is 900x900 px - your image will be bigger and sharper with this resolution.
Here is the preset for this size:
If you don't have images of this resolution, images that are at least 500x500 px are still fine.
Note that there is nothing wrong with sending a 900x500 image, for example. Just don’t make one side of it longer than double the size of the other side.
Also, note that images sent with Image Block appear differently on different devices. Below is some detailed information that might be not too interesting to know and also is subject to frequent changes.
When displayed on Messenger, an image can vary from 1:2 to 2:1 aspect ratio (width: height). Any image which is taller than 1:2 will be cut on its top and bottom so that only the middle part can be seen by a contact. Accordingly, any image which is wider than 2:1 will be cut on the left and the right sides, so that only the middle part can be seen.
When viewed on a desktop computer in a browser version of Messenger, wide images are shown as wide as a contact’s screen allows and, if too big, are downscaled preserving their proportions.
Tall images are cut at the top and the bottom if their height is more than 2.2x their width.
In the chat window on Facebook, an image is shown almost in the same aspect ratio as its original version. Very long or very tall images will appear very small, so they are strongly not recommended to use.
*Keep in mind it is stated in FB documentation 10Mb+ images cannot be shown in messenger, while they might be uploaded to the flow in most cases.*