Start by deciding what you want the plane to be capable of. Will it be bungee launched, hand launched or use landing gear.

Once you've decided on that, make an estimate of the ready to fly weight, including the fan and battery.

Then decide if you want vertical performance. If so, the thrust must be greater than the weight of the plane.

So if the AUW of your plane is 5 lbs and you want true vertical performance then the fan must produce at least 5.5 lbs of thrust. With that value in mind, your next task is to find a suitable fan/motor/ESC/battery combination

Or you can take the easy way out and use watts/lb. For good "jet" like performance figure on 175 watts.lb. While the resulting power/weight ratio may not permit true vertical, it will provide ample power to take off from a smooth runway.