VNS3 is a network appliance you run in your cloud environment alongside your application instances/VMs to provide network connectivity and security.
The size of the VNS3 instance/VM determines the access to the underlying physical resources, and the performance requirements of your network depend on your use case.
For production workloads we recommend an instance with at least 1 full virtual core, 3GB memory, and 30 GB disk - m4.medium in AWS, D1 in Azure or n1-standard-1 in GCE.
Optimal performance is usually gotten on the newest "generation" device, for example AWS c5.large+ is now the performance leader.
Greater network throughput will use more CPU, and additional container based services can use more CPU, RAM and disk.