Selecting ring sizes strongly depends on the VNF workload. It is expected that VNF vendors benchmark their VNFs to understand the optimal ring size configuration.
The VMXNET3 DPDK rings are intrinsic to VNF-C which means that CSPs may not be able to configure these parameters. Larger ring sizes provide an improved resiliency against the packet loss. However, using bigger ring size potentially increases the memory footprint which also potentially incurs a performance penalty.
Summary of Recommendations:
Use the NIC ring descriptor size with which packet loss is minimized.
Use ring sizes in ascending order through the traffic path.