<?php foreach( $posts as $key => $post ) : if ( $key == 2 ) : ?>
<div class="size2"></div>
<?php else : ?>
<div class="size1"></div>
<?php endif; endforeach; ?>
<div class="size1"></div>
<div class="size1"></div>
<div class="size2"></div>
<div class="size1"></div>
<div class="size1"></div>
<div class="size1"></div>
<div class="size1"></div>
<div class="size1"></div>
<div class="size1"></div>
<div class="size2"></div>
float: left;