I think a longer narrow board could definitely get you out winging in lighter winds than anything else. There would of course be a point of diminished returns which I'm guessing would be anything over6'6".
My mate is a bit smaller and very advanced paddler he has 2 sunova downwind boards that are around 6' x 22" and when he uses them for winging occasionally. He reckons they gather speed and get up on foil really easily and allow him to get out in super light winds. I have also heard reports of guys using the ONE egg pros with great results for light wind winging.
Bottom shape is a huge factor as well. My new custom sunova has really nice contours, it gets sailing speed and releases from the water really easy. My previous board was a ONE 6'5" recessed deck and it had a much rounder convex hull which pushed alot more water when winging and was harder to build speed to get on foil. ( still an excellent board though and to be fair it was designed previous to the winging revolution

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So my advice is just get one of the Tardrew brothers to make you a super sweet board and you will be loving life
