300,000 acres is a little less than 500 square miles (there are 640 acres to a square-mile section.) That's a piece of land roughly 20 miles wide by 24 miles long.
In comparative terms, it is almost half the size of the state of Rhode Island.
There is plenty of forest left in Arizona, but this is a pretty big fire. The biggest one I remember before this was about 30 or 40 thousand acres, if I recall right.
-ccm