For general slow flight 50 watts / lb is fine, for sportier flying 75-100 w/lb, and for 3d or powerful aerobatic performance 125-150w /lb is at least required (so you can point it up and climb vertically
I can only estimate at what you have there but given a 50 watt motor at peak power, I would say for satisfactory flying keep your weight below 8-10 oz TOTAL (including battery). That means you'll most likely be using a 2s-3s 400-600 mah lipo around 1-2 oz at most, with the plane and motor/electronics taking up the rest of the weight < 7-8 oz.
If you make a flying wing, that should be very easy to do, as all the weight is the wing itself and no fuselage or tail etc., to weigh it down with. 30-40" wing span should be doable.
I am going by the fact that I have also made a well flying delta wing plane, around 28", using a 20 gram 2200 kv motor and 3s 600mah lipo, 5" prop, with AUW around 8-9 Oz. The motor put out about 50 watts as well. While the plane is no 3D machine, it will fly and putt around on a calm day just fine, and even perform loops and lazy rolls.