There is a limit on UDP packet sizes that is much lower than the theoretical maximum of 64 KB.
All hosts must be prepared to accept datagrams of up to 576 octets (whether they arrive whole or in fragments). It is recommended that hosts only send datagrams larger than 576 octets if they have assurance that the destination is prepared to accept the larger datagrams.
There is no foolproof way to get the maximum size, it could be limited by OS settings, network hardware or various other things.
More info here: OSC blobs are lost above certain size