For an airliner'ish sort of flying mode you want to have roughly a .5 or .6 thrust to weight ratio. This will allow for decent but not jet fighter like climbs. SO if you know how much thrust the fan unit provides then you'll know how much total thrust you have and how much you should plan on the plane weighing. From there you can try to guess how large you can make it and still keep to that weight. The larger and more wing area the better. Let the fuselage size fall where the scale factor demands. There's nothing that says fat fuselages won't fly decently. Just make sure it's mostly air inside to keep the weight down to your target.