IO size is what is transported over the wire. It is typically application controlled. SQL/Exchange typically use 64KB IOsz. Your IOsz should be evenly divisible by your formatted blocksize but now a days might not have that much of an impact. Where it is important is when you are talking about SAN storage. A large IOsz that might be good for local disks can have a tremendously huge negative impact on SAN storage (Fibre Channel). So can very small. Go download the HPE Ninja stars tool and build a 3par/primera/alletra and look at the difference in service time and throughput for small IOsz 4K compared to 64K and 256K. You will have to multiply the IOsz by the IOPS for random workloads to understand the impact. Just as an example going form 64KB IOsz to 256KB IOsz gives you <7% increase in IO but a 3-4x increase in service times. Higher than 256KB it gets worse. You can usually tell what application is moving what IO based on the IOsz.