It's not entirely clear what you're asking here, so I'll answer both possibilities.
The length of the actual topic string is at most 65536 bytes. This is a limit imposed by the mqtt spec, you can't change it. It is also worth noting that the topic is encoded with utf-8, so you may have less than 65536 characters available.
The payload of the message is limited to 268,435,456 bytes. Again, this is defined by the spec.
If you are routinely approaching either of these limits you should be thinking about whether what you are doing is sensible.