At startup the client and server negotiate a maximum transmit size, which limits the size of nearly all SMB commands. You can set the maximum size that Samba will negotiate using the max xmit option in smb.conf . Note that this is the maximum size of SMB requests that Samba will accept, but not the maximum size that the client will accept. The client maximum receive size is sent to Samba by the client and Samba honors this limit.
It defaults to 65536 bytes (the maximum), but it is possible that some clients may perform better with a smaller transmit unit. Trying values of less than 2048 is likely to cause severe problems. In most cases the default is the best option.