Gallery thumbnails quality

Hello,

when I insert a gallery of images in my blog, it uses thumbnails of 150*150px, as indicated in media settings.

The problem is that when I add a gallery to my post, my template sets the width of each image at 300px. So the image appears pixeled. So I’d like to know how to use, for gallery, medium sized thumbnails (300*300px) instead of small sized thumbnails.

I’ve used this code and it worked, but now I’ve the problem that I can’t choice how many columns I want to use for each gallery, as these are fixed..

//Adds gallery shortcode defaults of size="medium" and columns="2"
function amethyst_gallery_atts( $out, $pairs, $atts ) {

    $atts = shortcode_atts( array(
        'columns' => '3',
        'size' => 'medium',
         ), $atts );

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

    return $out;

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