wordpress add field to post_class

I would first store your field value in a variable after your loop just before the posts HTML tags start:

<?php
$size = the_field( "size" );
?>

And then add this variable to the post class function:

<?php post_class($size); ?>

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