This article covers most of the basics:
http://soaringcafe.com/2011/01/desig...ion-sailplane/

Assuming you want to stay within the constraints of EASA CS22, wing loading can't exceed 60 kg/m2 much.

A higher aspect ratio not only means a higher L/D max, it also means that L/D max occurs at lower and lower relative speeds. In other words, going from AR=32 to AR=57, L/D max might go up substantially but fully ballasted, performance at 70 kts barely changes. For many long-ears L/D max already occurs at speeds one will only fly while thermalling.