It's for pecan bars, shortbread base, pecan pie topping.
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I think the 8 x 8 should work fine. Might be a thinner cookie in the 9x9.
9x13 is 117 sq inches in area. Divided in half is 59 sq inches. An 8x8 pan is 64 sq inches in area, so that's your best bet.
Awesome! Thanks for doing the math!