You can create custom thumbnail size using native WordPress method add_image_size()
And the size will appear under admin section as shown on the screenshot below.

Plugin Author
YARPP
(@jeffparker)
I’m not sure what to do here. I can find the thumbnail sizes, but they’re all square. How would I set a rectangular thumbnail. All of my images are landscape pictures of cars. Square thumbnails make them look pudgy.
rememberroad.com
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This reply was modified 3 years, 11 months ago by
carbuzzard.
You can define the image size like this
add_image_size( ‘car-thumbnail’, 300, 200, true );
This is just an example. So 300 is width, 200 is height, and true means the image will crop if it doesn’t fit in the aspected ratio.
Once that is done, you can set that size from Admin Setting as per the screenshot below.

Also make sure that once you add a new image size to your theme, you have to regenerate all the images to the new sizes. For that, you have to use the “Regenerate Thumbnails” plugin.
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This reply was modified 3 years, 10 months ago by
Aslam Doctor.
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This reply was modified 3 years, 10 months ago by
Aslam Doctor.
Sorry but I don’t understand something to custom the thumbnail image. I put this code in functions.php and block all my website with troubleshooting. So I missed something
So what is exactly the code to add to functions.php
Thanks for you time,
Manuel