pc-custom-image-size.php

function pc_theme_setup() {
	add_image_size( 'pc_image_custom', 300, 300, true ); // width: 300, height: 300 - feel free to change these values
}

add_action( 'after_setup_theme', 'pc_theme_setup' );

function pc_thumbnail_size( $size ) {
	return 'pc_image_custom';
}

add_filter( 'pc_apdc_thumbnail_size', 'pc_thumbnail_size' );