for NFS, it's basically the max and preferred read and write sizes
(512KB, 128KB, 512KB, 512KB resp on Isilon side). Just
make sure that clients do not limit requests to smaller sizes via the
NFS mount params rsize and wsize (as max possible sizes).
These might me set in the clients stab or auto mount map,
some systems might have other places to set global defaults.
Random small IOs will result in smaller request sizes, of course,
as they can't get coalesced on the client side.
SMB1 is limited to 64KB requests which always has been pretty bad,
while recent SMB allows for much larger requests. "Secure packet signing"
reduces the allowed request sizes though. For potential restrictions on the client
side, please refer to the Windows (or other clients OS, resp. Samba) docs.