Thank you very much for the work you do. Unfortunately, it is very difficult to know the area of an arbitrary shape knowing only its perimeter length. Consider a box with width 9 meters, height 1 meter. Then the perimeter is 20 meters with an area of 9 square meters. If the box is 5x5 meters, the perimeter is again 20 meters, but this time the area is 25 meters.
All I can do is provide you with a maximum upper limit to the area. This would be a perfect circle shape. If the perimeter of your burn swath is 64.8 nautical miles then the maximum area it could possibly have is 283,200 acres.
My guess is that it is probably around 3/4 of that, which is 212,400 acres. But this is pure guesswork.
If you have a map of your flightpath, say from GPS data, then there are ways to get that area much more accurately.