I was using my 5'10x 20x 85L sunova carver for over 20 knots Pwing. I am 105kg without wetsuit etc. The carver worked well and wasn't too difficult but does require power to get me going. In over 20 knots was quite nice but under about 18 not really do able. There just isn't enough initial power to get the board above the surface of the water. I could use the 5'10 carver in super light winds on the wing but not with the pwing.
I have since bought another board which will replace the carver. It's a custom Ben Tardrew/ code that I bought second hand from Hilly. It's 6'2x 21x 95L and it's much easier and more useable for me on the pwing. That bit of extra volume means the board is already almost at the surface and way easier to get going and generally a lot more stable. Thanks Hilly

I will use this in about 15- 18 knots and over I still have a 8ft downwind board I can use in light conditions.
If you're looking for a board for everyday decent conditions that will have a good wind range then a midlength around equal volume or -10 is about right. If you want to do downwinders super powered up you could get on something much smaller