Which model for which application?
Typically one will match the ID of the mains pipe to the building in question to the ID of the appropriate model.
Once the size of the mains inlet pipe exceeds the biggest model we would simply use multiple units in parallel connected to a manifold at the mains. The calculation would be achieved by simply dividing the cross-sectional area of the inlet of a particular model into the cross-sectional area of the mains pipe under consideration. For example a building with a mains pipe of ID 2″ would require 4x model 3 devices in parallel. This would ensure that there will be no flow restrictions.
We also always recommend a bypass system which any plumber can easily do for you.