The best-case for the Huguenots is probably that the Valois do not go extinct, so that Henri gives up the dream of being king of France and just tries to make the Kingdom of Navarre as big as possible. His personal success OTL was ultimately harmful to them in the long run as there was too much political advantage to him converting to Catholicism.
I voted for the "just Aquitaine" option but he probably would have some irregularly shaped territory covering a mix of Aquitaine, Béarn and Languedoc (but altogether, about the size of Aquitaine).
The other areas of Protestant concentration are probably lost and their residents would flee to Henri's kingdom, similar to how the Protestant Flemish fled north to the United Provinces. After this flight (and perhaps a simultaneous flight of Catholics out of Navarre) they might form a majority of the population. But they have two likely enemies in France and Spain...