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); ?>
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); ?>