Adding Post With Featured Image Programmatically

I am adding posts dynamically to wordpress. I am using following code to create post and add a featured image which is working fine but there are 2 problems

1. It adds 2 posts 1 with image itself no other content and correct with post with that image as featured image .

2. Featured image in post size large .

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   $post_id = wp_insert_post( $postdata, true );
   //Saving Image from a url
   $attachmentId_id = wp_insert_attachment( $attachment,$fullpathfilename,0);
   add_post_meta( $post_id, ‘_thumbnail_id’, $attachmentId, true );