rads doesn’t run on the host operating system.
But perhaps you mean how to assign more/less RAM to KVM VMs. That is undocumented for KVM. This is an imperfection in the documentation. Reported a bug here just now:
No idea when this will be done as I don’t maintain KVM. Meanwhile,this question is unspecific to Whonix. It’s the same for any KVM VM no matter if Whonix or non-Whonix.