Dels more the guru, im the apprentice. I use 6.5 ltr square pots which are great for a sog grow. Think dels going for the less is more approach, me id stick them in pot to pot and see how many would fit in that area and pick a strain that grows single cola and dont branch too much.
EDIT
just taken note of your tent size and its probably a bit much for a 250w to cover effectively, so with that in mind id say around 6 plants for a 250w cant say for sure as the lowest watt ive gots a 400w, ive got 8 under that so 6 should be ok for a 250