My first true learning experience with deep downwind slalom board sailing was about 20 years ago. Corpus Christi used to have their annual "Screaming Reach" which was a 10 mile downwind run the length of the bay.
As I recall, I was on a 120l slalom board and rigged a 6m sail and did a little reaching out and back to see if I had the right size. Good power and comfortable. However, once I started the race and tried to run as deep as possible, it quickly became clear that I was at least 1m to 1.5m too small on my sail choice. I quickly fell behind the more experienced local "bay racers" with much larger sails.
Live and learn!