Measure the diameter of the pot, then multiply that number by 3.147 and that will give you the circumference of the pot. one more step, divide that by two. exe, 13 inch diameter X 3.147 = 41 inches. divide that by 2 = 20 1/2 inch wide bag.
I know this is kind of late, but I ordered one from bagbrewer.com (after watching pod cast) and I just emailed him. He said give him the measurement across and the depth. I chose to give him the depth based on the false bottom so that the bag would not burn. I must say great quality bag for little money.