What size wire do you run to an air handler with 5kw heat

What voltage? How many phase? What distance?

Is this a new or existing unit? A new unit should come with installation instructions required by the code to be followed... it will usually also give the requirements for wire and overcurrent protection...
If used and unmarked then additional information is necessary...
Along with voltage, phase, and the distance you need to tell us what size motor is in the air handler. Then a reasonable guess can be made on the cable and overcurrent device size.