I have used NRF52840 and SDK 13.0.0. Module SPIM. Library does not support buffer size more then 8 bit. But I try write to register TXD.MAXCNT and I can't write value more, then 255. In datasheet size: 16 bit.
(nrf_spim.h):
__STATIC_INLINE void nrf_spim_tx_buffer_set(NRF_SPIM_Type * p_reg,
uint8_t const * p_buffer,
uint16_t length)
{
p_reg->TXD.PTR = (uint32_t)p_buffer;
p_reg->TXD.MAXCNT = /*length*/0x0000FFFF;
}