Airmaxx

Airmaxx stock their root pot here in six volumes, from small enough to raise a plant in up to fifty litres. Those six sit close enough together to pot a plant up in stages without jumping two sizes each time.

The size ladder, and what it means for watering

All six are under Pots and Trays. That page lists the volumes and explains what an air-permeable pot does to the rootball. Bear in mind when choosing that a pot like this dries faster than a solid one of the same volume, so the size you pick also decides how often you water.