Once upon a time, a long time ago, I was sailing in a dying wind on my big slalom board (AHD 284 - a very, very good light wind slalom board), with my powerful 6.5m Cam'd sail. The wind go so light that gradually everyone dropped off the plane and pulled in to the beach. Eventually I was forced in as well. One guy remained, planning back and forth. He was on a Starboard Formula 186 with an 8.5m sail. He came into the beach and asked me if I would like to try it. He didn't have to ask twice. He offered me the whole rig but I asked if I could try my 6.5m as it was set up for me. The wind would have been about 10-11 knots. I plugged in my sail, stepped on and two pumps later I was up and planning and doing it easy! I was amazed! Note that that it was impossible for me to get anywhere near planning on the big/light slalom board with the same sail.
Years later I bought a SH SB Formula 186 (now very outdated and not really competitive) for a very reasonable price.

I can tell you that you certainly do not need sails larger than 7.5-8m to easily get a 75kg sailor planning in around 8-10 knots and have an absolute blast.

As far as I can tell, the main advantage of the giant sails that Formula sailors use is to point high and fast upwind, and deep and fast downwind. If you are just cruizin', you will be well satisfied with the angles you can sail with a smaller sail, and save your back as well.

And so I lived in light wind land happily ever after.... well, that was until I rode a Tandem Formula type board (Gemini), but that is another tale kids.