how to get the post attachement image in full size?

Since you say that your image is “uploaded through add media”, you need to select the full sized image when you attach it to the post, not when you try to display it. You should even see in the markup that a smaller size has been chosen (by you).

Look over on the right side in the sidebar toward the bottom. You should see a dropdown that will let you choose “size”.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)