I don't think there is a value for increasing the are effect, you can try increasing END_RADIUS more than 15 and see what happens but remember to make a backup.
You can also edit the FORCE_FACTOR value which is the value that controls how much further cars and peds will fly from the explosion.
EDIT: I put the buzzard END_RADIUS to 50.0 and it worked just fine
This post has been edited by tomervutx123 on Sunday, Jul 8 2012, 10:23