WordPress + Isotope: how to give different widths to each entry?

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On the div that immediately wraps each post, you can use php to randomize the values instead of hardcoding the col-12 and col-sm-6 classes.

For example, you could change

<div class="col-12 col-sm-6 <?php echo $termsString; ?> grid-item wow fadeInUp">

to reflect the randomizer and then its result:

<?php
  $randomColClasses="col-" . rand(8,12); //random value between 8-12
  $randomColClasses .= ' col-sm-' . rand(4,8); //random value between 4-8
?>

<div class="<?php echo $randomColClasses . ' ' . $termsString; ?> grid-item wow fadeInUp">