[Plugin: Jetpack by WordPress.com] functions.php code for width not working

Forgive me if there is a post about this already but I’ve been searching and haven’t found an answer.

I’m trying to change the width of the Jetpack galleries on this website but the solution I found isn’t working.

I copied this code:

if ( ! isset( $content_width ) )
    $content_width = 630;

into functions.php but it doesn’t work.
I tried to find if there was a $content_width in the code somewhere but I can’t find it. I’m new at web design, sorry if this is an easy fix that I just can’t seem to figure out myself. I’m hoping someone could help me. Thanks!