Is the issue really about network "packet" (which is controlled by MTU settings -- default = 1500 bytes)? Or, about the size of the TCP buffer that FB allocates for sending/receiving msgs from the server?
About the size of the TCP buffer that FB allocates for sending/receiving msgs, i.e. TcpRemoteBufferSize.
But it does not matter for committed fix - as soon as server detects fragmented packet in async receiver it drops that attachment.