The theoretical limit (on Windows) for the maximum size of a UDP packet is 65507 bytes. This is documented here:
The correct maximum UDP message size is 65507, as determined by the following formula: 0xffff - (sizeof(IP Header) + sizeof(UDP Header)) = 65535-(20+8) = 65507
That being said, most protocols limit to a much smaller size - usually either 512 or occasionally 8192. You can often go higher than 548 safely if you are on a reliable network - but if you're broadcasting across the internet at large, the larger you go, the more likely you'll be to run into packet transmission problems and loss.