SPI and word size - what happens with (say) 8 bit and 24 bit data from the same chip?
It doesn't.  It just works in whatever data size you tell it to.

Typically that is 8 bits, though some can work in 16 or 32 bits for increased efficiency.

To transfer 8 bits you lower SS, transfer 8 bits, then raise SS.

To transfer 24 bits you lower SS, transfer 8 bits, transfer another 8 bits, then yet another 8 bits, then finally raise SS.



The entire transaction of 24 bits is just a series of 8 bit transfers framed by the SS pin being low.

What you then do with those three transferred values is entirely up to you.