You would change this in the php.ini file that your Mautic instance uses. It depends how you have set up your server as to which file would be the one to change.
You can find this by:
create a file called info.php that contains <?php phpinfo(); in the webroot (where Mautic is located), and run it in your browser (this is what you’d get in Mautic under System Info).
using the command (at command line)
php -i|grep 'php.ini'
A quick reference to where I explained the difference between local and master settings in case you need it: Asset size wont' change