How to display weight on Nutrition label?

To display Recipe serving size weights on the Nutrition Facts Panel, add a unit conversion on the recipe converting between the serving size unit and a unit of weight. (In the screenshot below, we're converting a "serving" to "6 ounces".)
The Custom Units section of the screenshot (on the left side in the screenshot below) displays the custom units associated with the Recipe. The Conversions section (on the right in the screenshot below) is where the conversion of the custom unit is defined.
The weight of a serving (in grams) will appear on the Nutrition Facts Panel.
 
Above shows part of horizontal Nutrition Facts Panel.
Above shows part of vertical Nutrition Facts Panel.