Help: Our Jenkins master node is too big, how to right-size?

Absolutely on point. That executor count metric is the pivot. The only nuance I'd add is that setting it to zero can be a shock to a team that's built habits around the master's executors. A more gradual approach is to limit the master to, say, one executor first. This forces the architectural conversation ("why is *any* build allowed here?") while preventing a hard stop in delivery. It turns a technical configuration into a workflow policy change.

Forget the cron job. That's just proving you have an expensive server.

The others have already told you the key metric: jenkins.executor.in.use.value. I'll add a concrete first step: go to Manage Jenkins -> Manage Nodes and Clouds. Look at your master node's configured number of executors right now. That number should be zero. If it's anything else, you've already found your problem and the instance size is irrelevant until you fix that.