There are no specific requirements for Logstash and Kibana, but keeping in mind a couple of things when designing an Elastic Stack is always a good approach.
Logstash and Kibana requirements
Logstash
Logstash is not heavy on CPU nor RAM, but this depends entirely on how many sources are feeding Logstash data since, for each event that Logstash parses, there is some overhead required to complete the process. If Logstash is to be installed on its own (with no other components on the same system), anything above one vCPU and 2 GBs of RAM should suffice for small/testing deployments. Ideally, the actual usage should be monitored and the system tuned accordingly. Logstash by default has in-memory queues that are used to store events...