I would keep it simple for now

includes/class-shortcodes.php

@@ -266,31 +266,6 @@ class Shortcodes {
 		return \sprintf( '<a href="%1$s">%1$s</a>', \esc_url( \wp_get_shortlink( $post->ID ) ) );
 	}
 
-	/**
-	 * Generates output for the ap_thumbnail shortcode
-	 *
-	 * @param array  $atts      shortcode attributes
-	 * @param string $content   shortcode content
-	 * @param string $tag       shortcode tag name
-	 *
-	 * @return string
-	 */
-	public static function thumbnail( $atts, $content, $tag ) {
-		$post_id = get_the_ID();
-
-		if ( ! $post_id ) {
-			return '';
-		}
-
-		$image = \get_the_post_thumbnail_url( $post_id, 'thumbnail' );
-
-		if ( ! $image ) {
-			return '';
-		}
-
-		return $image;
-	}
-
 	/**
 	 * Generates output for the ap_image shortcode
 	 *
@@ -307,18 +282,22 @@ class Shortcodes {
 			return '';
 		}
 
-		$atts = shortcode_atts( array( 'size' => 'full' ), $atts, $tag );
+		$atts = shortcode_atts(
+			array(
+				'type' => 'full',
+			),
+			$atts,
+			$tag
+		);
 
-		if ( is_array( $atts ) && array_key_exists( 'size', $atts ) ) {
-			$registered_sizes = wp_get_registered_image_subsizes();
-
-			if ( array_key_exists( $atts['size'], $registered_sizes ) ) {
-				$size = intval( $atts['size'] );
-			}
-		}
-
-		if ( ! $size ) {
 		$size = 'full';
+
+		if ( in_array(
+			$atts['type'],
+			array( 'thumbnail', 'medium', 'large', 'full' ),
+			true
+		) ) {
+			$size = $atts['type'];
 		}
 
 		$image = \get_the_post_thumbnail_url( $post_id, $size );

templates/settings.php

@@ -7,18 +7,6 @@
 		'welcome' => '',
 	)
 );
-
-$image_sizes = wp_get_registered_image_subsizes();
-$thumbnail_size = $image_sizes['thumbnail']['width'] . 'x' . $image_sizes['thumbnail']['height'] . ' px';
-$registered_sizes = '';
-
-foreach ( $image_sizes as $name => $size ) {
-	$registered_sizes .= $name . ', ';
-}
-
-$registered_sizes = trim( $registered_sizes, ', ' );
-$registered_sizes .= '.';
-
 ?>
 
 <div class="privacy-settings-body hide-if-no-js">
@@ -81,8 +69,7 @@ $registered_sizes .= '.';
 										<li><code>[ap_shortlink <i>type=xxx</i>]</code> - <?php echo \wp_kses( \__( 'The post\'s shortlink. I can recommend <a href="https://wordpress.org/plugins/hum/" target="_blank">Hum</a>, to prettify the Shortlinks. Type can be either: raw (the raw url, no escaping), esc (the html escaped url), html (default, an a tag to the url).', 'activitypub' ), 'default' ); ?></li>
 										<li><code>[ap_hashtags]</code> - <?php \esc_html_e( 'The post\'s tags as hashtags.', 'activitypub' ); ?></li>
 										<li><code>[ap_hashcats]</code> - <?php \esc_html_e( 'The post\'s categories as hashtags.', 'activitypub' ); ?></li>
-										<li><code>[ap_image <i>size=xxx</i>]</code> - <?php \esc_html_e( 'The URL for the post\'s featured image, defaults to full size. The size attribute can be any of the following:', 'activitypub' ); echo esc_html( $registered_sizes ); ?></li>
+										<li><code>[ap_image <i>type=full</i>]</code> - <?php \esc_html_e( 'The URL for the post\'s featured image, defaults to full size. The type attribute can be any of the following: thumbnail, medium, large, full', 'activitypub' ); ?></li>
-										<li><code>[ap_thumbnail]</code> - <?php echo \wp_kses( sprintf( __( 'The URL for the post\'s featured image thumbnail size (%s).', 'activitypub'), $thumbnail_size ), 'default' ); ?></li>
 										<li><code>[ap_author]</code> - <?php \esc_html_e( 'The author\'s name.', 'activitypub' ); ?></li>
 										<li><code>[ap_authorurl]</code> - <?php \esc_html_e( 'The URL to the author\'s profile page.', 'activitypub' ); ?></li>
 										<li><code>[ap_date]</code> - <?php \esc_html_e( 'The post\'s date.', 'activitypub' ); ?></li>