f_bsize appears to be linked to NFS's wsize parameter. You should be able to specify a lower value for that in the mount options on the NFS client.
Ideally, you'd set wsize to the server's physical block size. Higher values might result in (much) better write performance but the free space calculation will no longer be accurate.
Perhaps there's a clean way to figure out the correct block size for NFS mounts, but I'm going with what pacman currently does, statvfs(2).