Courtney, the recipe calls for an 8" x 8" pan, which is 64 sq. inches. Double that, 128 sq. inches. 9" x 13" = 117 sq. inches, while 11" x 13" = 143 sq. inches. Looks to me like the 9" x 13" is a bit closer fit for doubling. As for baking at altitude, you might want to skim through our high altitude baking tips. But given there's no leavening in these, I don't believe you're going to have to make any adjustments. Enjoy - PJH

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