The storage requirement for the Prometheus database for storing Skyline data is roughly 25MB per reporting device for the default retention period of 15 days. This default can be changed in the Prometheus server configuration, and you should change the calculation accordingly.
We don't have exact numbers for the CPU and RAM requirements, but I can tell you that we have tested a couple dozen reporting devices to a Prometheus&Grafana server installed on a little VM with 4 cores and 8GB of RAM. For larger environments, I would advise getting something a little more robust, but you can see that it doesn't have to be too big.