Hi! Thank you very much for your support and interest in the plugin! ^^
There isn’t really a “recommended size,” since that will depend on the card design you create and want to use. You should be able to use images in any size and adjust them using the features provided by the Studio. That said, I recommend avoiding images that are too large or too heavy, so they load faster and provide a smoother experience. I also recommend using JPG format to keep file sizes lower while maintaining good compression.
Please feel free to contact me if you have any questions or run into any issues.
Best regards!