Anything above 3KB and less than 3MB in length is split into 3 chunks using self encryption. Greater than 3MB then it is split into 1MB chunks using self encryption.
This means that to have a single chunk for a file then it has to be less than 3KB
Now it has been discussed that somebody could self encrypt a file independently of the network and use the chunks generated to know the hash (XOR) address of the chunks and then see if they are there.
This would mean in theory someone could find if a particular file has already been uploaded by checking the XOR addresses of the chunks.
Some even suggested that this is a way they could save themselves the PUT cost of files that have potential to already be uploaded. (eg a popular cat video)
Apart from that it is pretty as much as @polpolrene said.