Hi, I need to ask how big a flowerpot I should use...

Your medium and watering habits play a role too.. you can grow a very large plant in a small hydroponic system relative to a pot with soil in it of the same size. in this case "hydro" includes soilless mediums like coco too. how long you vege a plant matters. 12-12 from seed and you don't need much. 60 day vege and you probably want a 5-7 gallon (19L-26.6L ish). i've also read 1-gallon per 1 foot of vertical height as a reccommendation. This seems to be rational for smaller pots. I have some donors in about 1/2gallon of coco and they grow to ~16" tall or more before they hit their temporary ceiling due to a small pot. So, that math is just meant to be safe and not accurate or precise. ~4gallon pot... i'd guesstimate a 30"x30" to 36"x36" plant is all you'd want before roots can't support the size well... assumes light is strong enough too.

Depends on what strain you are growing. For Autos I always go for around 10 liters, photoperiods need more depending on how long you are going to let them veg. Good luck!

15L is perfect. Can get descent tops from 7L but for any good yeild 15L or more.

All depends on how big a girl you wanna grow my friend I use 20 litre pots and my girls can go to 5 ft tall 4 ft wide indoors. Cheers