Caesalpinia is not something grown very much here in the UK so I am taking advice from a book. It suggests a light trim after flowering and cutting back shoots that are badly placed. As you want to reduce the size of the shrub, I would suggest cutting about one third of the branches out completely, then giving a good feed. Repeat for the next couple of years as well, by which time you will have removed all the oldest wood and the shrub will be reduced in size. By spreading the task over three years, it is not such a huge shock to the plant.

We grow lots of Caesalpinia here in the desert! The pruning technique depends on the species:
C. gillesii can be pruned back to stumps if it is relatively young--less than 5 years old--and will grow back to near mature size within a year or so. Older trees should be pruned back in stages, with 1/3 of the branches cut back each spring. We usually prune before bloom, though that delays bloom, it reduces sunburn of recently exposed branches. That may be less of a problem in Southern France.
C. pulcherrima (probably rare except in the very warmest spots) is routinely cut back to 15 cm stumps every spring. Often, winter frost does that for you, though you still have to remove the deadwood!
C. mexicana can easily take a variety of pruning strategies, including hedge trimming, or minimal pruning to let it grow into a lush tree. Timing isn't critical with this species, except I wouldn't prune too close to fall, which would increase the chance of frost damage.