OnApp Storage supports block sector sizes as small as 512 bytes, however the system's optimal size is in multiples of 64K.
In order to leverage stripe parallelisation you should operate with outstanding IO in the queue of at least #stripes x 256K.
To test the optimised performance for a sequential IO test like dd a blocksize should be larger than #stripes x 256K.
From the VM level and filesystem most of the templates use 4096 bytes.
The default block size is 64KB.