What is the optimum size of cable duct for prestressing?

The cross sectional area of duct is normally 2.5 times that of the area of prestressing steel. The size of ducts should be not designed to be too small because of the followings:

(i) Potential blockage by grout
(ii) Excessive development of friction
(iii) Difficulty in threading prestressing tendon

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.