My grow box is a little bigger than yours, 21"x 35"x 60" (6 sq ft) and according to a grow guide I downloaded, I needed a 400 Watt HPS or MH light for this size area. Here's what the guide says for general rules regarding lighting: Light is measured in Lumens 3000 lumens per square foot for min. light req. 5000 lumens per square foot for med. light req. 7000 lumens per square foot for max. light req. Or in Watts: 30-50 Watts per square foot in HPS/MH lights 150-250 Watts per square foot in Fluro lights Your grow area is about 4 square feet so you would probably do best with at least a 250 Watt MH or HPS light. MH is better for vegetative, HPS for flowering but if you can only afford one, go with the HPS. I have both in a switchable system.
MH is short for Metal Halide, HPS is High Pressure Sodium. These are two fluoro bulbs that use high intensity discharge ballasts. The regular fluoros don't have high intensity discharge (HID). The MH has a blue/white spectrum which is ideal for vegetative growth whereas the HPS has a red/orange spectrum, better for the flowering phase. If you can't afford the HID lights above and go with regular fluoros it would probably be better to invest in two or more (at a lower wattage) to spread the light over a wider area and crisscross the light pattern. I'm serious about my crop so I've invested in the 400 Watt HPS/MH switchable and a good strain of Indica seeds from the Netherlands. It's a good idea to line the walls of your grow space with a reflective surface (mylar/aluminum foil) or paint white.
If your lights are regular fluoros and not HID then that wattage isn't enough (you probably won't get to the flowering phase). I don't know what your grow space size is but you may want to get a couple of 150 or 250 watt regular fluoros and box in your grow space with reflective walls for maximum lighting (and a fan for air circulation and heat reduction).