The setting for the Width animation property defines the thickness of the pipe. Typically, you would not animate the Width property—you would set the width on the Pipe Properties window. However, you can animate the property if required.
The Width property requires a floating number, so you can enter any number for the width (the higher the number, the greater the thickness). Alternatively, you can use the Expression window's browse feature to associate the Width property with the value of a database item.