Hi kalaiselvan:
I am also a user of the CM403F,and I already use the SPI to control other chips.In my opinion ,your understanding is not right.I think FIFO size is fixed of 64 bits,so "the size is 4 if word size is 16 bit" it means if your receive data word size is 16-bit,the FIFO can buffer 4 words of this data.So the number of the FIFO can buffered is decided by the receive data word size.But the maximum is 32bits.
Hope this can help you.
Best regards
HLG