What is the minimum and maximum diameter that waterCAD allows to use to model a network. I mean theoretical diameters without considering commercial diameters
There is no any theoretical minimum and maximum pipe diameter limit in WaterCAD, you can model pipes of any size and WaterCAD will design your system for pressure, velocity, headloss based on that.
You can try that yourself, prepare different scenarios for different diameters and see the results and compare for a better solution.
Scenario and Alternative Management
Bentley Technical Suppport
Sushma is right in that there is no limit in WaterCAD. However you do need to be aware of the limitations in the underlying equations you are using to calculate head loss. The Hazen-Williams and Manning equations are based on turbulent flow so if you are working with tiny pipes and a very viscous fluid, you are better off using eth Darcy-Weisbach equation in the calculation options. But for typical water system piping, any of the equations provided in WaterCAD should be acceptable.
Thank you so much