I have a Centos 7 KVM VM with only one purpose - to run Red Hat Cockpit. I ran it today and it had a static use of 256 MB RAM - even as I added other servers for it to watch and clicked all over the web interface. It currently has 1 GB RAM as that's what libvirt defaults to for a CentOS 7 VM. I was thinking of shrinking it to 512 MB to give it a bit of overhead just in case. When I check the CentOS website, I see minimum RAM listings for x86 on Centos 6.9, but not for 7. If it's reporting only using 256, do you foresee any issues with me running it with 512?

Other details:
It was installed from the CentOS 7 mimimal disc and is commandline only - no desktop environments.

Try it and see.
Keep an eye on swap usage (in the guest) as a warning sign. When I cared about such things in the past I would reduce the guest to the point where it just started to swap. Adjust the size up to give it a bit of headroom and leave it alone unless things changed in the future. This is not bullet proof of course, but does catch scenarios where start-up uses more memory than is obvious from just looking at the stable running system.
Automatic ballooning is the ultimate answer, but I'm not sure of its current status in KVM.

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OK, thanks. I did a bit more Googling and it seems that installation requires 1GB. Outside of that, people reported having trouble running yum on 64MB RAM. That's a bit drastic. It's just that, in my understanding you can over-commit a CPU, but not RAM. So I don't want to have my VMs use more RAM than they actually need. Your idea about swap is a great one. I'll mark as solved.