If CAP_LARGE_READX or CAP_LARGE_WRITEX capability is enabled on the SMB Negotiate Server Response, the maximum buffer size used is 61440 (60K) for large read( SMB_COM_ READ_ANDX ) and 65535 (64K) for large write (SMB_COM_WRITE_ANDX) , regardless of MaxBufferSize. But this is only true if the SMB signing is not turned on
But this has nothing to do with the "best stripe size", it is just a network protocol specification. The determination of the "best" stripe size would be based on your data read and write patterns. For large, sequential reads and writes you would chose a large stripe size. Small random I/O requests would ask for a smaller stripe size.
You can monitor the Avg. disk bytes / operation physical disk perfmon counter to get a rough idea about the typical request size of your I/O load.