There isn't clear rules on image dimension and how it will display on Messenger mobile and desktop. I'm trying to display an image on generic template, but the image cropped to half, is there a way to display a thumbnail and display a full version on click?
I was running exactly into the same issue. Seems you can modify image dimensions by changing the image_aspect_ratio to "square". The default is "horizontal".
Further documentation you can find under: https://developers.facebook.com/docs/messenger-platform/send-messages/template/generic#fields
"Square" and "Horizontal" are the only options though. Everything else needs to be solved via web-view!