How do I size a zener diode for use as a freewheel diode for a ZVS induction heater?
24V zeners are liable to be far too low in voltage rating.
At 28V a 24V zener will be drawing "immense" current. 

Your 24V zener draws 10 mA at 24V - dissipation is already about 50% of rated continuous.
At 28V current may be 100 mA++ - 2.8 W dissipation and magic smoke country. 

Place a say 0.1 ohm or even 1 ohm in series with a zener to allow you to measure the current it is passing.
At 10 mA a 1 ohm would drop 10 mV - what do you get during normal operation? 

This datasheet  is not your for your device but close enough - Look at fig 3 on page 3!!!

This high quality precision extrapolated  diagram shows a 22V zener operating at 26V - 4V above its rated voltage. Your 24V zeners if operated at 28V will fare similarly. For a while .