About resizing the RSS image

Dear developers,

I have tried to resize the RSS image to 500 * 0, how can I do it? I have tried investigate in your code but I don’t think my coding knowledge is enough to understand it. I have also tried to use the code you provided in description, yet it did not work.

function thumbnail_size_for_rss( $default ) {
return ‘thumbnail’;
}
add_filter( ‘rfi_rss_image_size’, ‘thumbnail_size_for_rss’, 10, 1 );

Thank you for reading and your help.

Yours faithfully,
Hugo Sum