How do I size the coupling capacitance for a SEPIC converter?
To make the switching effects couple from input to output, we need
  enough capacitance to overwhelm any parasitic capacitances on the
  output side of the converter.


If the two inductors are coupled then a SEPIC can become a flyback converter and the capacitor is not needed. This tells you that there is no minimum value for the coupling capacitor: -



I think it's worth linking to this article where I stole the picture. 

The article is a called "Power supply topology: SEPIC vs Flyback" published by Electronics Weekly.

Basically and in simple terms you don't need the capacitor if using coupled inductors but, the more capacitance you apply the more like a SEPIC converter it becomes with the various trade-offs you'll get.