im_udp SockBufSize option
SockBufSize
is for the socket buffer and specifying this explicitly should help with the issue in theory. I think the default value is set by the OS.
Note that the standard UDP packet size is limited to 64k. See the wikipedia page on UDP for example. Even if you manage to generate larger UDP datagrams, these might get discarded elsewhere on the network by routers due to the above reason. Also note that as per RFC3164 the The total length of the packet MUST be 1024 bytes or less.