A very general rule of thumb is that your garden needs 50 watts of high intensity discharge (HID) lighting per square foot of illuminated area. Such as a Metal Halide (MH) or High Pressure Sodium (HPS).
However, this rule ignores the shape of your marijuana grow room so the following is really a better guide:
A 250 watt HID will illuminate a 2' x 2' garden.
A 400 watt HID will illuminate a 3' x 3' garden.
A 600 watt HID will illuminate a 3.5' x 3.5' garden.
A 1000 watt HID will illuminate a 4' x 4' garden.
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.
Light diminishes rapidly the farther you go from the bulb. Each bulb has a limited range, beyond which you do not have good marijuana plant growth.
For example, a grow room that's 2 x 10 feet would require 1000 watts using the 50 watts per square foot. However, a 1000 watt bulb only covers an area about 5 feet across - meaning the edges of your marijuana grow room will be darker. The light is not spread equally across the whole room. Which is the goal of the marijuana grower.
A better choice in this case would be three 400s or two 600s.
I could go on, but that's a basic guideline for lighting a grow room for good marijuana plant growth using common HIDs.
Quality seeds require a well lit grow room to properly grow and flower cannabis. Consider equipment costs when starting a marijuana grow room before purchasing pot seeds.
Invest in the best seeds and equipment to create amazing conditions to grow cannabis in.