Consider a single lenses focusing a laser. Two properties we might be interested in are the diameter of the waist and the lenght of the confocal region at the waist. For example, if we are running the beam through an iris, perhaps as a spatial filter, we will need to know the waist size. In practice, the minimum waist size may be limited by the damage threshold of the crystal so for this example we will be concerned with the confocal length. Furthermore we will ignore the index of refraction of the crystal and consider a gaussian beam propagating in air.
Lets decide to use milimeters as our unit of length.
The program does not care what units you use, however all lengths must be specified in the same units. A common mistake is to mix units such as entering waists in microns, wavelengths in nanometers, and focal lengths in millimeters. Set the wavelenth and beam size as follows. Click on the first rectangle, and up pops an adjusment panel in the lower corners of the diagram. This is the most common situation encountered with lasers.