Is it possible to install a HA harvester cluster with 2 nodes with ample resources and a 3rd small witness node that does not participate in Longhorn or run VMs? If so, how do you set up the install configuration to install a node without longhorn storage

Short answer is no. Long answer is Harvester will be installed the same on all nodes, it's just the first 3 nodes, regardless of spec, will be management nodes. Longhorn is req'd for Harvester. If you had another 2 witness nodes you might be better off installing Harvester on 3 witness nodes first then adding your ample nodes.

Thank you; I believe I understand. Perhaps witness node was not exactly the correct terminology. Let's assume your scenario above; can the first three nodes be configured so that harvester will not launch vms on them or longhorn will not place data on them? Ie is there a way to provoke harvester into separating the k8s+harvester control plane nodes from kubevirt+data nodes? Ie setting taints?

I just reused your witness node term as it sounded lower spec. Harvester is all Kubernetes underneath so if it's not possible via "GUI" then it should be possible using kubectl commands. What you don't want to do is stop Harvester from working but there will be a way.