How do I select a heater for size 5 or larger starter with CTs?

If the heater charts are available, those do the math for you - meaning select the heater shown for the FLA of your motor. Those charts have already factored in the CT ratio.  If the heater table is not available, the required heater or solid state overload relay rating can be calculated by dividing the FLA of the motor by the current transformer (CT) ratio. For example: if the motor FLA is 365amps and the starter has a 600:5 ratio CT, take 600/5 = 120, then divide 365amps by 120 = 3.04 amps.  Select the heater and set the overload relay dial to 3.04 amps.  This math applies to heater type overload relays as well as fixed heater type (XTOB) or SSOL (C440 or XTOE).