There is no maximum length of a UDP segment, since it can be fragmented across multiple IP datagrams.

Since the UDP header length field is 16 bits and indicates the number of bytes in the UDP segment, a segment can be as large as roughly 2**16 bytes

Since the UDP header length field is 8 bits and indicates the number of bytes in the UDP segment, a segment can be as large as roughly 2**8 bytes

1024 bytes.