I would recommend 4K, and about 2/3 Read and 1/3 Write and about 80% random. The biggest thing though - you have to disable Windows caching to get accurate results. Why? As far as I know, ESX doesn't cache SAN data including iSCSI. Now I'm not sure about this, but until I considered this, the IOMeter results were opposite to what we were actually experiencing on ESX.