This setting determines the size of the circle drawn by Design Master Plumbing to represent a vertical pipe. All pipes will be drawn at the size indicated here, regardless of actual dimensions. A 4" pipe and a 1" pipe will both be drawn at the same size. To set this size, select:
DM Plumbing 
Customization Options Edit Standards List
The Design Master Plumbing Options dialog will appear.
The Vertical Pipe Drafting Method has been set as Not To Scale. The Minimum Vertical Pipe Diameter option has been disabled out and cannot be set.
Indicate the size that you want to draw pipes. The number you indicated represents inches on the printed drawing. A setting of 0.1 means that your pipes will be drawn such that they appear 1/10th of an inch in diameter when printed.