I am running a 400 watt in a 4x5 closet flowering 3 lil ladies, two of them are in three gallon pots and one of them is in a five gallon.
As far as the room and light, I feel that i could get more plants in the room with those pot sizes. I also feel that if i were to grow more plants i would need a 600watter.
but that's just how it goes i guess. It's all up too you.