Where is the template from the_post_thumbnail()?

Assuming that this is a theme that you can edit– ie. one of your own creation– the_post_thumbnail accepts an argument for size.

the_post_thumbnail( $size, $attr );

Just give it the size you want, for example:

the_post_thumbnail('full');

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