Source

File: wp-includes/media.php

function wp_image_src_get_dimensions( $image_src, $image_meta, $attachment_id = 0 ) {
	if ( ! wp_image_file_matches_image_meta( $image_src, $image_meta, $attachment_id ) ) {
		return false;
	}

	// Is it a full size image?
	if ( strpos( $image_src, $image_meta['file'] ) !== false ) {
		return array(
			(int) $image_meta['width'],
			(int) $image_meta['height'],
		);
	}

	if ( ! empty( $image_meta['sizes'] ) ) {
		$src_filename = wp_basename( $image_src );

		foreach ( $image_meta['sizes'] as $image_size_data ) {
			if ( $src_filename === $image_size_data['file'] ) {
				return array(
					(int) $image_size_data['width'],
					(int) $image_size_data['height'],
				);
			}
		}
	}

	return false;
}