How do I size a Networker Storage Node?
I found no recommendation in the documents.
The sizing for the internal PCI bus and Network are clear, but what about CPU and Memory?
This entirely depends on your architecture used and specs you expect. There is straight answer to that and when it comes to sizing of that kind, you will have to rely on HW tuning docs and not software application one. With that said, once upon the time we had few discussions here about oldish formulas for both CPU and memory, but I can't say how relevant they are nowadays (as those were focussed on older architectures on SPARC machines for example).
You can find a couple of interesting docs in Powerlink: Performance Optimization Planning Guide (P/N 300-013-559) and IOPS Requirements for the Networker Server (P/N 300-012-592). The second one is referred to NW Server, but I think you can apply some rules for your Storage Node.