From the datasheet of the only OctaSPI flash we currently support
(http://www.mt-system.ru/sites/default/files/docs/Macronix/mx25lm51245g_3v_512mb_v0.01.pdf):

After program/erase command is issued, auto program/erase
algorithms which program/erase and verify the specified page
or sector/block locations will be executed. Program command is
executed on byte basis, or page (256 bytes) basis, or word basis.
Erase command is executed on sector (4K-byte), or block (64K-byte),
or whole chip basis.

So the minimum write size is one byte.