There is a few factors that are used to determine the size of the phase converter to be used, we have to determine the load type and horsepower and is there any resistive loads.There is different load types, easy, medium, hard, very hard, frequent starts, and resistive loads. With an easy load we can use the same size phase converter as the motor you are running, medium we use one size up, hard we double the size, and very hard 2.5 times more. resistive is determined by the amps.
Easy Loads - Should be at one size larger than load motor.
Medium Loads - Should be 1.5 times larger then load motor.
Hard/Heavy Loads - Should be double (2 times) the size of load motor.
Very Hard/Heavy Loads - Should be 2.5 times larger than load motor.
This link will help you choose the right size phase converter for your application. https://www.phoenixphaseconverters.com/sizing.php