viz. man mkfs.xfs:
-b block_size_options
This option specifies the fundamental block size of the filesystem. The valid block_size_options are: log=value or size=value and only one can be supplied. The block size is specified either as a base two logarithm value with log=, or in bytes with size=. The default value is 4096 bytes (4 KiB), the minimum is 512, and the maximum is 65536 (64 KiB).
To specify any options on the command line in units of filesystem blocks, this option must be specified first so that the filesystem block size is applied consistently to all options.
Although mkfs.xfs will accept any of these values and create a valid filesystem, XFS on Linux can only mount filesystems with pagesize or smaller blocks.