Oh, I understood that the smallest possible block size is of the order of 10MB.
The standard TTree basket size is 32kB, but in practice individual reads requests are often combined and therefore larger. I’d suggest 128kB to start with, but in fact I’d be very interested myself if you see a difference in performance for, say, 32kB or 1MB (I made a note to myself, that’s something we should benchmark). Another point to consider is the hardware block size. I think the file system block size should ideally be a multiple of the block size of the drive.