More info is always better but i'll try to explain it better. Pot size depends on the type of grow you are doing and how long you want to veg the plant(s). The longer you veg the bigger the root system gets. The bigger the root system, the bigger the pot needed. Also the bigger the root system, the bigger your plant can get. I prefer vegging my plants for 6 weeks then flipping as that is usually the amount of time needed for me to train the plant out to my desired width. Some dont want to veg that long and do micro grows, which is basically letting the plant get it's first set of true leaves then flipping to 12/12 (or doing 12/12 from start). Since the plant wont get very large because of short veg time you can add a lot more small container plants to fill up that same space. I have a 3x3 space that i put 4 3-5 gallon pots in. That same tent i could do 12-18 plants in one gallon pots. Trained correctly 4 plants can produce as much as the 12-18 plants, though it takes a longer veg time to fill out the tent.
If you still have problems you can definitely post pics and info.