Tap the gold swap button on a meal in My Viva and use the Size row: R Regular, L Large (+£2, about 50% more by weight), P+ Protein+ (+£2, a Regular with extra protein) or LP+ Large Protein+ (+£4). The price difference shows before you confirm.
A size change applies from that delivery onwards and updates your plan, so your next charge reflects it. Not every dish comes in every size; the sheet only offers the ones it does.