How can I edit the backplane exchange size on the MVI56-ADM?

First use the MVIcip_open function to open the backplane then you will need to use   'MVIcip_WriteConnected' function call and the 'MVIcip_ReadConnected' function call to change the I/O size.   The last argument of the these two functions (MVIcip_WriteConnected and MVIcip_ReadConnected) is the dataSize argument in size of bytes.  The detail on how to use these two functions can be found on page 273 of 348 and page 274 of 348 on the adm_dev_guide respectively.