We have a couple of general recommendations you can follow but we also recommend testing any images you use across multiple devices to find what fits best for you.
With iOS, you can use an image up to 1,038 pixels wide by 1,038 pixels or any smaller number, generally, landscape images look better.
With Android, the image display varies between devices as it will be scaled to fit the notification center, however a ratio of between 2:1 and 16:9 should give the least amount of crop and best look and feel but again, testing is advised.
Please let me know if you have any other questions or concerns.
Urban Airship Technical Support