functions.php

<?php
/**
 * Add Column Classes to Display Posts Shortcodes
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/add-column-classes-to-display-posts-shortcode
 * 
 * Usage: [display-posts columns="2"]
 *
 * @param array $classes
 * @param object $post
 * @param object $query
 * @return array $classes
 */
function be_display_post_class( $classes, $post, $listing, $atts ) {
	if( !isset( $atts['columns'] ) )
		return $classes;
		
	$columns = array( '', '', 'one-half', 'one-third', 'one-fourth', 'one-fifth', 'one-sixth' );
	$classes[] = $columns[$atts['columns']];
	if( 0 == $listing->current_post || 0 == $listing->current_post % $atts['columns'] )
		$classes[] = 'first';
	return $classes;
}
add_filter( 'display_posts_shortcode_post_class', 'be_display_post_class', 10, 4 );

style.css

/* Column Classes
------------------------------------------------------------ */
 
.five-sixths,
.four-fifths,
.four-sixths,
.one-fifth,
.one-fourth,
.one-half,
.one-sixth,
.one-third,
.three-fifths,
.three-fourths,
.three-sixths,
.two-fifths,
.two-fourths,
.two-sixths,
.two-thirds {
    float: left;
    margin: 0 0 20px;
    padding-left: 3%;
}
 
.one-half,
.three-sixths,
.two-fourths {
    width: 48%;
}
 
.one-third,
.two-sixths {
    width: 31%;
}
 
.four-sixths,
.two-thirds {
    width: 65%;
}
 
.one-fourth {
    width: 22.5%;
}
 
.three-fourths {
    width: 73.5%;
}
 
.one-fifth {
    width: 17.4%;
}
 
.two-fifths {
    width: 37.8%;
}
 
.three-fifths {
    width: 58.2%;
}
 
.four-fifths {
    width: 78.6%;
}
 
.one-sixth {
    width: 14%;
}
 
.five-sixths {
    width: 82%;
}
 
.first {
    clear: both;
    padding-left: 0;
}