In open source, we need to manage our settings with our own module. Luckily, camptocamp/puppetserver provides exactly what we need! We can use this module to create a profile that applies to our Puppetservers:
class profile::puppetserver { class { 'puppetserver': config => { 'java_args' => { 'xms' => '4g', 'xmx' => '6g', 'maxpermsize' => '512m', }, } }}