How to Size a Cyclone or Hydrocyclone has a well establish step-by-step process for which Richard A. Arterburn was a pioneer.  Since this paper and its hydrocyclone sizing method is becoming hard to find, I uploaded it on here.

Use the 911Metallurgist Online Cyclone Design Calculator.

Hydrocyclone Design & Sizing Parameters – Calculations & Equations

I also include an Excel file prepared by Alex Doll from SAGMilling.com to help you properly calculate the size of a hydrocyclone.

Further credit FLSmidth

Discussion classifications by screens and cyclones.