Don't worry about the storage block size. Size your database pages according to your needs (e.g. 256KB). The 'average' IQ write size might be around 128KB but could be anywhere from 1 IQ block to 16 (16 to 256K from this example).
Size storage according to the throughput required - currenly suggested around 50-100MB/sec/core (thats megabytes). Not IIOPS. Depending on your IQ page size, that could be 25K IIOPS per core (at the usual 4K per IIOP metric of the vendors). Sustained.
That is irrespective of the storage block size after all.
Another metric is to try have 2-3 mechanical spindles per core. (Some drives are only able to acheive 30MB/sec).
You will also want to spread IQ devices over many spindles. Try not to place IQ file devices on different partitions across the same spindles/LUNs as this can cause contention as well.
Please refer to the IQ Sizing Guide at:
http://scn.sap.com/docs/DOC-46166
for more detail.
Chris