50 litres is like what? A 15 gallon pot? Those are for some monster plants! General rule of thumb is you was 1 gallon for every foot of growth.
Check out @MIMedGrower grow logs for an idea. He uses 3 gallon pots and the plants get quite large.
First step is a long and health veg, and then once you have a strong plant it's time to flower.
Key to a healthy harvest is a healthy veg.
15 000 lumen leds are not all that powerful. A 600w hps puts out 60 000-90 000 lumens and is able to cover a 4ftx4ft area.
Area coverage will depend on your light coverage. Try to find out how many watts you light drawing, and that should tell you how much space you can cover, which will in turn determine the size your plants will be.