Twelve by twelve should cover you and then some. Your ridgeline length on a 10' 6" hammock is going to be in the 8' 9" range. A 12' tarp would give you about a foot and an half clearance on either end of the hammock. As far as the width, I have the Hennessy Hex (twelve long by ten wide), and it's more than spacious.
Since you seem to be fairly handy with a sewing machine, have you considered doing a DIY tarp? A simple rectangle or square doesn't require tricky cutting, and there are several sources here on the board to consult for tarp basics. DIY Gear Supply has 1.1 sil as low as $5.50 a linear yard. Note that this stuff may have cosmetic imperfections in it. That being said, it's hard to beat the price.
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