Thanks! Changing rsize/wsize removes the disk space check error.
For posterity, including myself:
Indeed, from what I could gather from the archived discussions: http://lkml.iu.edu/hypermail/linux/kernel/0310.1/0392.html (and the subsequent message by Linus) http://h30499.www3.hp.com/t5/LVM-and-VxVM/definition-of-f-bsize-vs-f-frsize/... it seems that f_bsize originally meant "high-level block size for efficient I/O" which explains why it's linked to rsize/wsize.
I was about to suggest f_frsize ("fragment size") as an alternative, but a quick test reveals that it has the same value as f_bsize, on that very same machine/NFS; which confirms Linus' suggestion for "optimal block size everywhere" in the discussion linked above. BTW I have checked the "one file per block" rule by successfully creating more than 10000 files from that same client/NFS, so it seems that this result is somehow allowed by the abstraction.