You can select a dynamic sizing method for the piping for the pipe systems in a project using the Pipe Sizing dialog. 
Note: The Pipe Sizing dialog cannot be used to size Sanitary system piping. 
- Select the sections of pipe being sized. 
 
- Click Multi-Select tab
Analysis panel
 Duct/Pipe Sizing.  
- In the Pipe Sizing dialog, select a sizing method from the drop list: Velocity, or Friction. Specify one of the following options associated with the selected method: 
- Only to size pipes according to the parameters for the selected method (Velocity or Friction) exclusively. 
 
- And to force sizing pipes to meet the parameters that you specify for both Velocity and Friction. 
 
- Or to allow sizing pipes according to the parameters of either Friction, Velocity, or both.