@Crypto_Fireside feature image aspect ratio varies across different themes, but for Casper a regular 16:9 should work. Bear in mind that if you use images that shouldn’t be cropped (e.g. diagrams) then you may bump into issues with that.
If you are using featured images where cropping is an issue, Casper actually has a dedicated layout that you can use. If you go to the Design settings page, under Homepage → Feed layout, and select the “Grid” option.
Read more about automatic image resizing. We also recently added a new img_url helper, which has not been added to Casper yet but it’s on our radar.