4. Grow – Barney's Farm – Photo - SanLight – BioBizz

If you use photoperiodic seeds 11l is enough so you have to water all2-3 days but if you run automatics the Motto is bigger is better but i think 20l pot is ok

20 Litres minimum.

That's fine as long as you stick to 4-5 week vege. Up to a 4 month old plant will fit in that pot without crowding roots. "They" often say 1month per gallon... or, 1 foot of vertical height per gallon... After chopping them down, you can inspect the substrate and see if the roots are overgrown or not. If you go bigger, you can see if there is a lot of wasted substrate with minimal root growth etc... In a 4x2 space, 3 plants won't need an excessive vege time, so i'm sure 11 L will be fine. Even 2 plants (slightly longer vege to fill space) is probably fine. I've been using "5 gallons" (which are 3.6gal actual volume... not sure if they measure the same way, over here it's 5gal 'compressed' or 'trade gallons?' i think). I have plenty of roots around the outside, but could probably use slightly smaller. I stick to 5-6 week vege then your typical flower length give or take a couple weeks. My plants take up a bit more space than yours per plant, so most likely even more room for your roots with a slightly shorter vege and slgihtly smaller full grown plant -- 1/3rd of 4x2 vs 1/2. Whether soil or soilless might make a difference too. If in soil and get a little slower growth, maybe go up 1 size. Like i said, whatever you do this time, you can inspect the substrate when you pull it out of the pot and see if portions are lacking roots or over-crowded etc.