The easiest way to think about it is when you are growing marijuana you will need about fifty watts of HID lighting per foot. This assumes that your garden is square, try to use this as a principal.

1 250 watt two foot by two foot garden.
1 400 watt for a three foot by three foot garden.
1 600 watt for a 3.5 foot by 3.5 foot garden.
1 1000 watt for four foot by four foot garden.

All you have to do to make that general rule work is line your walls with flat white paint or mylar to make sure your garden gets ample coverage, also try to make sure the lamp has a good reflector on it.

That should get you started