Should be an easy answer - if you want to use a custom thumbnail on an assets folder (Open the folder properties) - what are the ideal dimensions / size so that it will maintain aspect ratio
This is the feature I am looking at:
Checked with an available example - The OOB coral card dimensions are 318*229. The image to be uploaded should be in similar aspect ratio for best results, otherwise the card container will adjust it accordingly.
Dimensions should be as Gaurav mentioned above.
however if you want to make any custom change. then please go through with below file. you will get more info.