While I was prepping all the shear webs, and showing Eric how I do stacks of them at once, Tim was busy making patterns for the wing sheeting.
Now I must admit that because I was in my small workshop with Eric sanding stacks of shear webs on my belt (and disc) sander, I wasn't there in the main workshop to watch Tim at work. But, I believe he used the dimensions of the wing as drawn on the plans nearly exactly (just very slightly oversized to allow for slight misalignments when gluing) for the bottom sheeting. The top sheeting requires a little more allowance in size (beyond the dimensions as drawn on the plans) because the airfoil is semi-symmetrical; therefore the top sheeting must curve around this airfoil. This requires a little more allowance for length from front to back.
(Please chime-in, Tim, if anything I am assuming is in any way inaccurate; or, you can offer some extra details that builders need to consider in these steps.)