Use shortcode_atts_gallery filter to add new defaults to the WordPress gallery shortcode. Use image size of medium and 2 columns as new defaults. Allows user input to the post gallery shortcode.

filter-gallery-shortcode

<?php
/* Use only code below

/**
* Use shortcode_atts_gallery filter to add new defaults to the WordPress gallery shortcode.
* New gallery shortcode defaults (columns="2" and size="medium").
* Allows user input in the post gallery shortcode.
*
* @author Marcy Diaz
* @link http://amethystwebsitedesign.com/how-to-use-larger-images-in-a-wordpress-gallery/
*/

function amethyst_gallery_atts( $out, $pairs, $atts ) {
   
    $atts = shortcode_atts( array(
        'columns' => '2',
        'size' => 'medium',
         ), $atts );

    $out['columns'] = $atts['columns'];
    $out['size'] = $atts['size'];

    return $out;

}
add_filter( 'shortcode_atts_gallery', 'amethyst_gallery_atts', 10, 3 );