Changing playlist shortcode thumbnail sizes?

Demo Plugin – Fixed Size

Here’s one suggestion as a demo plugin (PHP 5.4+):

<?php
/* Plugin Name: Custom Playlist Thumb Size */

namespace WPSE238646\PlaylistThumbSize;

add_shortcode( 'playlist', function( $atts = [], $content="" )
{
    add_filter( 'wp_get_attachment_image_src', __NAMESPACE__ . '\\src' , 10, 3 );
    $out = wp_playlist_shortcode( $atts, $content );
    remove_filter( 'wp_get_attachment_image_src', __NAMESPACE__ . '\\src' );    
    return $out;        
} );

function src( $image, $id, $size )
{
    add_filter( 'image_downsize', __NAMESPACE__ . '\\size', 10, 3 );
    return $image;
}

function size( $downsize, $id, $size )
{
    remove_filter( current_filter(), __FUNCTION__ );
    if( 'thumbnail' !== $size )
        return $downsize;   
    return image_downsize( $id, $size="large" );           // <-- Adjust size here!
}

Here we change the thumb size from 'thumbnail' to 'large'.

First we override the playlist shortcode callback, to better scope our filters and only target the playlist shortcodes.

Then we hook into the image_downsize filter to our needs, to call the image_downsize() function with the wanted thumb size. But we must remember to remove our filter callback right away to avoid an infinite recursive loop.

Demo Plugin – Various Sizes

But it would be more flexible if we could write the thumb size as a shortcode attribute:

Here we prefix our _size attribute with an underscore, just in case it gets supported by core in the future.

Here’s a modification of our first demo plugin to support that (in a compact form):

<?php
/* Plugin Name: Playlist With _size Attribute */

namespace WPSE238646\PlaylistThumbSize;

add_shortcode( 'playlist', [ (new Playlist), 'shortcode' ] );

class Playlist
{
    protected $_size;

    public function shortcode( $atts = [], $content="" )
    {
        $allowed_sizes = (array) get_intermediate_image_sizes(); // <-- Allowed sizes
        $this->_size="thumbnail";  // <-- Default size
        if( ! empty( $atts['_size' ] ) && in_array( $atts['_size' ], $allowed_sizes ) )
            $this->_size = $atts['_size' ]; 
        add_filter( 'wp_get_attachment_image_src', [ $this, 'src' ] , 10, 3 );
        $out = wp_playlist_shortcode( $atts, $content );  // <-- Native playlist
        remove_filter( 'wp_get_attachment_image_src', [ $this, 'src' ] );
        return $out;       
    }     
    public function src( $image, $id, $size )
    {
        add_filter( 'image_downsize', [ $this, 'size' ], 10, 3 );
        return $image;
    }
    public function size( $downsize, $id, $size )
    {
        remove_filter( current_filter(), [ $this, 'size' ] );
        if( 'thumbnail' !== $size )
            return $downsize;  
        return image_downsize( $id, $this->_size ); // <--Apply new size
    }
} // end class

Here we restrict the allowed sizes to get_intermediate_image_sizes().

Hopefully you can test this further and adjust to your needs!