Hello there UK Green. The general rule of thumb when it comes to growing cannabis is only one plant per pot. Then your container size must match gallons versus height of the plant. I don't know the metric system very well so I'm going by standard american engineering here. Say your cannabis plant was three feet tall then you would have to have a three gallon cointainer to grow it in to allow the roots space to grow without encountering nutrient lockout in which the plant has taken up all of the space availible to it and asorbed all of the nutrients out of the medium. To make it more simple remember one gallon per foot of the plant. Here's an american site for nursery containers you can view. http://www.midwesthydroponics.com/pr...p?category=260
They are very inexpensive to purchase. I hope I was of help to you and good luck brother.