Generating very small sized thumbnails from featured image

I can’t speak for the size of the image, but I assume small images that have a dimension about 50px should have the size around 5 or 10kb. You can add your own custom size to your theme:

add_image_size( 'tiny-thumbnails', 50, 50, true );

Setting the fourth parameter to true will force the WordPress to crop the thumbnails, so they will be exactly 50px in width and 50px in height.

Now, calling this function will get you your tiny thumbnails:

the_post_thumbnail_url( $post_id ,'tiny-thumbnails' );

Don’t forget to regenerate the thumbnails after adding this code to your theme’s functions.php file.

You can also tweak with the JPG quality, to reduce the size of your thumbnails (which also reduces the quality!). The jpeg_quality filter allows you to do this.

// Set your quality to any value you wish. Values below 70 are not recommended
add_filter('jpeg_quality', function($arg){ return 85; });