Photos in the generic template that aren't 1.91 (width) to 1 (height) aspect ratio will be scaled or cropped. It’s the same ratio across Messenger and Facebook chat box.
The maximum size is 909x476 px, which makes your media appear flawless on any device.
NOTE: If you don't have images of this resolution, most users won't probably notice the quality drop at all, if the image is at least 500x262 px (minimum size).
If an image is closer to a perfect square, the better. This way it appears larger and more prominent.
The maximum size is 900x900 px - your image will be bigger and sharper with this resolution.
NOTE: If you don't have images of this resolution, images which are at least 500x500 px (minimum size) are still fine.
NOTE: Images sent with Image Block appear differently on different devices. It is stated in Facebook documentation that 10Mb+ images cannot be shown in messenger, while they might be uploaded to the flow in most cases.