Set Custom Size Featured Image in Wordpress

Add below the code in your finction.php file ( here two thumnail size use )

add_image_size( 'featured_thumbnail20x76', 120, 76, true );

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 Use below the Code in your post loop

the_post_thumbnail( 'featured_thumbnail326x132' );

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Regenerate the Old Media Uploaded image Use belo the Plugin to Regenerate

https://wordpress.org/plugins/regenerate-thumbnails/screenshots/


After Plugin Intalation and Resize all Images ( above mentioned image size)

Simple and Great...!