Your wattage will totally depend on your own setup and ambient temps.
As an example (totally made up numbers). If you had a 2x2x4 ft screen enclosure and to get your basking spot to 85 F you needed a 75 watt. I also have a 2x2x4 screen enclosure, but because my ambient temperature is 5 warmer, I only need a 40 Watt.
You need to figure out the wattage by checking the temperatures with either a Temperature gun (most accurate) or digital thermometer. If using a digital thermometer it needs to stay in that spot for 15-20 minutes before giving a close to accurate reading.
What kind of chameleon do you have?