How to change Jetpack's tiled gallery width

Using a child-theme is the best option when edition your themes files.

Code for functions.php

if ( ! isset( $content_width ) )
     $content_width = 600; // set this to whatever width you need to suit your content area e.g 960

Code for style.css

.size-auto, 
.size-full,
.size-large,
.size-medium,
.size-thumbnail {
     max-width: 100%;
     height: auto;
}

Source: http://iamgw.com/wordpress-plugins/change-jetpack-tiled-gallery-width/