It ain't easy to determine a list with the biggest waterfall if you are talking about the volume (amount of water) in a waterfall. I can read and speak a little bit French but not enough to do proper scientific research. I also don't know if there are figures about the flowrates of rivers with waterfalls. 

Nevertheless I made a top 10 biggest waterfalls in France based upon a visual examination. There is a lot of photo material and I have been several times to France to visit the most impressive waterfalls.

Enjoy my list "top 10 biggest waterfalls in France"

If you said, in forehand, that Grande cascade de Gavarnie is the waterfall with the biggest flowrate, I would say "really????". But if you walk closer to the base of the waterfall you will see how mighty this waterfall is. With a (maximum) flowrate of 200m3/sec in summertime this could be the biggest waterfall in France.