1. Go to Settings > Kitchen Settings > Portion Sizes (Portions module). The page has a tab per portion type - Meal Prep and, with the Catering module, Catering - each listing ID, Name and Active/Inactive status. The row marked (Default Portion) is the base size every meal has.
2. Click Add portion to Meal Prep (the button follows the selected tab).
3. Enter the Name, optionally a Default Container Type (used for meals on this portion unless overridden per meal-portion) and keep Is Active? on. Super admins can also change the Type.
4. Click Save.

Active portion sizes appear as extra price rows (with their own ingredient quantities and macros) in every meal editor, as filters in the Meal Recipes list, and as columns on the Meal Prices grid. Deactivate a size to hide it from new meals without deleting it.