With the Portions module, every active portion size you created under Settings > Kitchen Settings > Portion Sizes appears as a row under the default portion in the Prices table. For each one:

• Turn on the Active toggle to offer that size for this meal.
• Upgrade Price is the increase from the default portion - if the default sells for $10 and Large should be $12, enter 2.
• Surcharge, Serving Size, Container and Manual Production Price work like the default row.
• Catering portions show a Minimum Quantity column instead of Regular Price.

Each active portion also gets its own ingredient quantities and macros (see the ingredients section), so a Large can really contain more food. Portions you switch off are removed from the meal on save, together with their quantities and macros.