If you know the KV & amp rating, I’d look for a motor with similar ratings & try the prop it uses. If you don’t know the ratings of your motor, you will need to sneak up on it. Start small prop, run it for 5 seconds see if it’s hot. If not run it for a minute & check temperature. If it not hot, do it again with a larger prop. Warm is ok, but if you can’t hold the motor in your hand for 30 seconds, it’s too hot. You need a smaller prop.