How to secure 20 lb light fixture install?
Toggle bolts. Put them in the drywall on either end. There's no chance it will pull out.

Let me expand on this by telling you a true story. I bought a house that had been reposessed by the bank. I can only assume the previous owners ransacked the place (even the fireplace mantle had been removed) and the bank had hired a contractor to come in and replace all the light fixtures. So he put a new fluorescent light (not dissimilar to yours) up in the kitchen.

With screw-in drywall anchors (insert duhn-duhn-duhnnn here).

This is what my wife walked into one morning not long after we moved in.



The screw-in anchors are rated for the weight, but not the parallel downward force (they're designed for perpendicular force). So I promptly went to the local big-box and bought some toggle bolts. Since I now had two brand-new holes, I only needed to slightly enlarge them to get the toggles through. That fixture hasn't moved since.

The toggles are your best bet because they


Distribute the weight of the anchor across a wider area
Would require the complete destruction of the drywall to pull out


Even the smallest toggles are rated far more than your fixture weight (a single 2" toggle is typically rated for 50-70lbs).

So what size do you need? Any size toggle will do, just be sure to check the size of the head. It should cover the hole on your fixture easily, but you might need a washer if the holes are large enough.