For a normal (Preset) plan, click the Portion Size value in Renewal Settings and pick a size or Any Sizes; it saves immediately and applies to newly created dates and the meal picker. For a Custom (bundle) plan the value lists each category with its mix (e.g. "1 x Medium, 1 x Large"); clicking opens Portion size per category, where steppers set how many meals of each portion each category gets. Each category's counts must equal its meals per day (the tally shows "N of M assigned" and Save stays disabled until every category adds up). Saving rewrites the mix on the subscription and on every open delivery date that has no meals selected yet. Each delivery date also has its own Portion Size field in the Subscription Dates panel; it is only editable while the day has no meals selected, otherwise you see "To change the settings of the day of this subscription you must refund the selected meals first".