I understand you are aware of how to design a cyclone following the equations of e.g. Barth and Muschelknautz which are the basis for a proper design. However, those equations do not give any hints about common relative proportions of cyclones.
Here are some rules of thumbs which can be found in literature (VDI Berichte 1511, p. 25):
Diameter cyclone / diameter central tube = 2
(Total length cylcone - Length central tube) / diameter central tube = 5 to 10
Length central tube / diameter central tube = 1
In the literature mentioned by m777182 iterative equations for optimizing the length of the central tube can be found which take into account geometry, viscosity, density and velocity in the cyclone.