The numbers work together if you have a 208 volt, three phase heater. Assigning the current a value of 53.4 takes into account an extra 25% for its being considered a continuous load. Therefore, you need a wire with an ampacity of at least 53.4 amps, and need not oversize it beyond that point. So a #6 copper THHN would answer, since it has an ampacity of 65 amps. A #8 would not work, since it is rated only for 50 amps. You cannot ?undersize a wire?; it must have an ampacity at least as high as the load. Of course, if the branch circuit is a long one, over 100 feet, you might want to consider upsizing for voltage drop considerations.
The breaker need only be rated to protect the wire. A 60 amp breaker would do. Since the wire has an ampacity of 65 amps, and since that is not a standard size overcurrent protection setting, you can upsize the breaker to the next step, and use a 70 amp breaker. You can always undersize a breaker. You could protect this installation with a 15 amp breaker, if you wish. I don?t think the owner would be happy with the results (i.e., it will trip all the time), but that is not a concern of the NEC.