There really is no hard and fast rule about plane geometry. It rather depends on what you want the plane to do and what you make it from.
Model gliders tend to have fuselages around 1/2 the wing span where as many fast or aerobatic designs have a wing span the same as the fuselage length. 75% is a reasonable average.
A JasonK points out the rear of the fuselage acts as lever. The stability of the plane is effected by the effectiveness of this lever. The longer the rear fuselage the smaller the tail plane (or fin) needs to be to generate the same lever action.
The material the rear fuselage is made of is likely to limit how long the fuselage can be without it becoming too heavy. A carbon fibre tube for example can be very slender and still be rigid enough.
Aerodynamics and plane stability are very complex subjects and there are a great many variables at work so following the proportions of an existing design is always a good starting point but deviate too far from it and the plane may become difficult or even impossible to fly.
A 5 x 5 prop sound very small for a 1 kg plane. With a 2 m span and a 10:1 aspect ratio made of 5 mm Depron it is unlikely to be structurally capable of flying fast so you would be better with a low Kv motor (1000 or even less) turning a bigger (10 x 3.8?) prop slower. It would also be considerably more efficient at converting the Watts into usable thrust.
I presume your big battery is to obtain endurance.
This layout might give you some ideas.
60" (1520 mm) span. A 'built up' (Clarke Y section) wing from 3 mm Depron with a balsa spar. Fuselage 6 mm Depron with a glass fibre tail boom. A 3000 mAh 3s driving a 950 kV motor turning a 9 x 3.8 prop. At 565 g all up it was a slow flyer but could climb vertically at full power. At minimum cruising power it had an endurance of over an hour. It could glide (& thermal) pretty well too.