Look at models online and determine what motor and battery are being used for airplanes the size you are talking about.
Tower Hobbies is a good place to start.
http://towerhobbies.com/
The model research will give you a good idea of motor, battery and electronics weights as well as overall aircraft weight based on building material.
Research wing loading and try to design the plane at 20 oz./sq.ft. or less.
By already determining the appropriate motor and battery you can then set a size limit and weight for the airplane so the student can concentrate solely on airframe design.