the shard size is currently being tested in the beta phase, among other configurations. The end users will be able to request the level of redundancy they prefer, however, I the chunk size would not be a user defined option in order to assure network efficiency and data privacy.

Exerpt from the Storj whitepaper:

"We define a standardized shard size as a byte multiple such as 8 MB or 32 MB. These are kept at pre-set sizes to dissuade any attempt to determine what is being stored. Sharding also allows large files to be more manageable as they are distributed throughout the network..."
 
You can find out more about the details of chunking on Storj in the whitepaper´s chapter 2 here: http://storj.io/storj.pdf