For your 12/12 hour grow with six clones in a 60x60 cm box, I recommend pots with a volume of 5 to 7.5 liters.
The 5-liter pots are the most suitable. With six plants in such a small space, 5 liters is big enough for nutrient supply and small enough to fit all the plants and avoid overcrowding the grow box.
If you use 7.5-liter pots, it will already be very tight.
Optimal range: 5 liters to 7.5 liters per plant.
In a 60x60, 6 plants are already relatively tight (equivalent to about 16.6 plants per m2). 5-liter pots are a good choice as they still provide enough nutrient buffer but don't take up too much space, allowing you to accommodate the 6 clones better. 7.5-liter pots would also work, but it will be very tight, and the risk of overcrowding increases.
Since you are working with clones and switching them to 12/12 immediately (or after a very short growth phase), the plants will stay smaller than if you had a long vegetative phase. Smaller plants need smaller pots.
Hope that helps you out! 😘