1. Make sure Order: Subscription > Meal Plan: Portion Size per Category is set to Yes (it is the default).
2. On /admin/plans click Add new and choose Custom plan in the Create a subscription plan pop-up (or open the form and set Portion Model to Customize).
3. Fill in the plan as usual — the single Portion Size field is hidden for Custom plans — and pick a Group that contains no Preset plans.
4. Save, then click Add New Pricing. Besides days, meals, delivery days and categories you now get Portion size per category: for every category that has meals, enter how many of them are each portion size; the tally must read e.g. "5 of 5 assigned". Leave a category at zero to use the plan's default portion size.
5. Save the price. The Portions column of the table shows the mix (e.g. "Entrees: 3XLarge, 2XRegular") or Default.

You may add several bundles with the same days × meals but different portion mixes.