When sizing a downstem there are three main sizing factors to consider:

Length - Length is measured from the top of the joint to the bottom of the downstem

Diameter - Diameter is measured at the top of the joint across its width

Bowl Size  - If you have a preexisting bowl piece or a preferred size then keep this in mind when you order. 

The inner diameter of the downstem's ground joint is the same size as the slide it takes. So an 18/14mm downstem takes a 14mm slide, an 18/18mm downstem takes an 18mm slide and a 24/18mm downstem takes an 18mm slide.