Kouger, my first thought is to search out 3 view drawings on the net for other sailplanes and get your inspiration for the fuselage and tail sizes from them.

If in doubt making the fuselage longer and the tail larger by a small fraction will only make it fly better but at perhaps the very slight cost of a small amount of increased drag from the small extra amount of area you build in.

There's nothing like doing some good natured plagarizing.... er make that "research" when designing your first model of a given type. Just be sure to match the tail lengths to the areas. In other words don't steal the short tail length off one model and the small tail area off another. Stay consistent. The safest method is to find as many drawings as you can and average them. Be aware that the aspect ratio of your wing also plays a part. Steal from designs that are close to your wing aspect ratio.



Puffmaru-
That comment was totally uncalled for.