I'm still a little stumped. I searched for a while and only find 6 watt LEDs mounted or in bulbs. However, I can tell from your specs that you read the label to me and you are really not an electronics person. Apparently you have a power supply and a LED that are seperate and you want to make sure the LED doesn't smoke.
So, I guessed a 25 degree centigrade rise for 6 watts and that comes out to about 4 centigrade degrees per watt. Then I looked in my catalogs for a heat sink that would do that, and I came up with 2 inches by 2 inches. That's how I did it and that's the answer. At least 2 inches by 2 inches of good, thick metal (at least 1/8th of an inch) is required to radiate the heat and give the LED a long and happy life. Good enough?