i think id get it into a larger pot.You will want more light in the grow..if it stretch's to much you can bend the plants top over and tie it down .
The first thing id do is transplant it into a larger pot.Around 10 gallon pot or 40 litres.Go to 12 / 12 and get you some more lights.
A guy on here grew a massive plant in a closet just like yours with LED lights..cree bulbs to be exact..you can find that at lowes and mennards if you have them in your country.
He was using 188 watts of LED cree bulbs is all.Some of the bulbs were mounted over the plant with several mounted in the closet aimed at the side of his plant.
The strain he was growing was called critcal kush.
How mush stretch the plant has depends on what kind of strain it is..afghanis tent to not stretch as much and indicas..Sativas stretch like crazy.