Index: src/wp-includes/general-template.php
===================================================================
--- src/wp-includes/general-template.php	(revision 37081)
+++ src/wp-includes/general-template.php	(working copy)
@@ -876,19 +876,13 @@
 	}
 
 	$custom_logo_id = get_theme_mod( 'custom_logo' );
-	$size           = get_theme_support( 'custom-logo', 'size' );
 
-	if ( ! $size ) {
-		$size = 'full';
-	}
-
 	// We have a logo. Logo is go.
 	if ( $custom_logo_id ) {
 		$html = sprintf( '<a href="%1$s" class="custom-logo-link" rel="home" itemprop="url">%2$s</a>',
 			esc_url( home_url( '/' ) ),
-			wp_get_attachment_image( $custom_logo_id, $size, false, array(
-				'class'     => "custom-logo attachment-$size",
-				'data-size' => $size,
+			wp_get_attachment_image( $custom_logo_id, 'full', false, array(
+				'class'     => 'custom-logo',
 				'itemprop'  => 'logo',
 			) )
 		);
@@ -896,9 +890,8 @@
 
 	// If no logo is set but we're in the Customizer, leave a placeholder (needed for the live preview).
 	elseif ( is_customize_preview() ) {
-		$html = sprintf( '<a href="%1$s" class="custom-logo-link" style="display:none;"><img class="custom-logo" data-size="%2$s" /></a>',
-			esc_url( home_url( '/' ) ),
-			esc_attr( $size )
+		$html = sprintf( '<a href="%1$s" class="custom-logo-link" style="display:none;"><img class="custom-logo"/></a>',
+			esc_url( home_url( '/' ) )
 		);
 	}
 
@@ -912,9 +905,8 @@
 	 * @since 4.5.0
 	 *
 	 * @param string $html Custom logo HTML output.
-	 * @param string $size Size specified in add_theme_support declaration, or 'full' default.
 	 */
-	return apply_filters( 'get_custom_logo', $html, $size );
+	return apply_filters( 'get_custom_logo', $html );
 }
 
 /**
Index: tests/phpunit/tests/general/template.php
===================================================================
--- tests/phpunit/tests/general/template.php	(revision 37081)
+++ tests/phpunit/tests/general/template.php	(working copy)
@@ -279,11 +279,8 @@
 
 		$this->_set_custom_logo();
 		$home_url = get_home_url( $blog_id, '/' );
-		$size     = get_theme_support( 'custom-logo', 'size' );
-		$size     = $size ? $size : 'full';
-		$image    = wp_get_attachment_image( $this->custom_logo_id, $size, false, array(
-			'class'     => "custom-logo attachment-$size",
-			'data-size' => $size,
+		$image    = wp_get_attachment_image( $this->custom_logo_id, 'full', false, array(
+			'class'     => 'custom-logo',
 			'itemprop'  => 'logo',
 		) );
 		restore_current_blog();
@@ -302,11 +299,8 @@
 		the_custom_logo();
 
 		$this->_set_custom_logo();
-		$size  = get_theme_support( 'custom-logo', 'size' );
-		$size  = $size ? $size : 'full';
-		$image = wp_get_attachment_image( $this->custom_logo_id, $size, false, array(
-			'class'     => "custom-logo attachment-$size",
-			'data-size' => $size,
+		$image = wp_get_attachment_image( $this->custom_logo_id, 'full', false, array(
+			'class'     => 'custom-logo',
 			'itemprop'  => 'logo',
 		) );