It is not possible to control the packet size with these units as they service the serial port data on a polling basis. You can lower or increase the polling rate using the optimize setting:
This can be configured using telnet or SSH to the connect to the unit, logging in as root and issuing the following command:
#> set config optimize=latency
This will increase the polling rate (putting higher load on the network).
#> set config optimize=throughput
This will decrease the polling rate.