Hi folks, some of you seem to have gotten side tracked.
40 liters = 10.5 gallons.
The pots he's using are really nice large pots as you can see in some of the pics.
10.5 gallons is an absolutely GREAT size for a container.
Good luck to you noelk. Those plants are looking great!
Sometimes, it's helpful to those who are looking at photos if you put a measuring stick or a pack of cigs next to your container. Relative size is obvious to everyone that way.
If you wish to top your plants and grow the cuttings into clones, I would suggest that you follow these simple rules:
1. Take a 4 to 6 inch cutting. That's 10 to 15 cm.
2. Trim all but two of the largest fan leaves from it. This, as leelow says, will slow transpiration down and let the plant concentrate on root growth instead of trying to support a bunch of leaf.
3. Have your container ready. It should have no light in the area that the roots will develop.
4. Hold the cutting under water and make the last cut to it's stem on a 45 degree angle. This opens the inside pulp of the stem as much as possible for water intake and by doing this under water, you prevent the chance of an air embolism that could block the passage of water into the plant.
5. The water mix should contain rooting hormone and a good anti-fungal.
Roots should appear in about 10 to 15 days and grow rapidly.
That's it man. If you do this correctly, you'll have thriving, healthy plants.
Good luck to you man!