Configure packet size when sent from WR21 serial port?

I am sending and receiving data on the serial port of a WR21 using UDP over the air. The Serial Port is breaking the data into 128 byte packets, but I would like to use the full 512 bytes available in UDP. Is there anyway to change this?

I seem to have found a way around this problem by setting the S15 Data Forwarding Timer to a value that is higher than 2. It appears that the device attached to the serial port is being interrupted and the WR21 takes that interruption as and end of data timeout.