Horizontal stab size has nothing to do about fuselage shape (other than tailboom length). It's all about ability to winch launch. If you skip that, you can put very small one to your glider. Concordia uses 27 stabilator and I bet they have no plans for winch launches.

Schempp will install self-launching Solo to bigger Ventus fuselage, there will be no possibilities for tight waist because of that. It will most likely be Ventus2c/Quintus -type fuselage.