According to TCP/IP Illustrated, the max IP datagram is 65535 bytes, which means that you should be able to get 65507 bytes of user data in a UDP datagram.

But Rich Stevens found that kernel bugs sometimes impose other limits.

If you are using sockets, you can try a setsockopt() to increase the buffer size. Use both SO_RCVBUF and SO_SNDBUF. You can probably raise your limit somewhat this way. But you may max out at 32K.

Don't worry about ethernet. The IP layer will fragment the IP datagram as required. You will not exceed the max ethernet frame size no matter what you do.