Watch this.
https://youtu.be/-PYzKgw9-Ps
The actual target area on this ability is massive. The hitbox isn't necessarily the problem, its that the animation doesn't actually display how ridiculously wide the actual hitbox is.
At the very least, the devs should give it an area indicator on the ground as she casts it, similar to the change they gave Irelia's E recently. If I could just see how far I need to move before I get blown up, I would be satisfied.
Personally, I think the best option would be to give it two hitboxes; a thinner center hitbox that does more damage than it currently does, with the wider area it is now doing less damage. This would reward Lux players who are very skilled or who land their full combo on somebody (since the thin center area is easy to line up on a rooted target), but it would prevent her from just throwing it out into a choke or a teamfight and getting huge damage/kills because of the forgiving area.