A very general rule of thumb is that your garden needs 50 watts of HID lighting per square foot of illuminated area.
One 250 watt HID will illuminate a 2' x 2' garden.
One 400 watt HID will illuminate a 3' x 3' garden.
One 600 watt HID will illuminate a 3.5' x 3.5' garden.
One 1000 watt HID will illuminate a 4' x 4' garden.
Contributed by Jackerspackle, here's a different take on watts-per-square-foot (WPSF).
WPSF assumes that the bulb's intensity is equal over the entire grow area.. in other words each square foot/meter receives the same number of lumens. But in reality, light diminishes rapidly the farther you go from the bulb (1/4 the intensity for each doubling of the distance from the light source). Therefore, each bulb has a limited range, beyond which you do not have good growth.
For example, a growspace that's 2 x 10 feet would require 1000 watts if you go by the 50 WPSF guideline that's commonly mentioned.. But a 1000 watt bulb only covers an area about 5 feet across - meaning the edges of your garden will be dark.. A better choice in this case would be three 400s or two 600s.
Another problem with WPSF is that it assumes all bulbs have the same intensity. But 1000 watts of HPS is not the same as 1000 watts of fluorescents or (yuk) incandescents. Fluorescents have their lumens spread out over a long tube and are therefore dim.. incandescents have the wrong color spectrum and are also dim.
Nor is a 1000 watt HPS the same as four 250 HPS lights. 250s don't have anywhere near the intensity needed to penetrate thick canopy or tall, bushy plants. (So don't even think about growing meter-tall plants with a 250).
Here's a basic guideline for lighting a growspace for good growth using common HIDs:
WATTAGE -- COVERAGE 1000 watt - 4 to 5 feet across (1.3 to 1.5 meters) 600 watt - 3.5 feet (1 meter) 400 watt - 2.5 possibly 3 feet (.8 to .9 m) 250 watt - 2 feet (.6-.7 m) 175 watt - small, less than 2 feet
These numbers assume you have a good reflector around your bulb and also reflective wall coverings. You can increase the figures a bit if using multiple bulbs, due to their overlapping effect. You can also increase coverage using a light mover that will physically rotate your light around your grow area.