What size should the aluminum pan be?

If you use an Internet search engine to seek out Kenji's recipe at Serious Eats

http://www.seriouseats.com/recipes/2017/02/detroit-style-pizza-recipe.html

you'll find a hyperlink to an Amazon.com store item for an authentic Detroit pizza pan:

https://www.amazon.com/LloydPans-Kitchenware-Hard-Anodized-Detroit-Style/dp/B01FY5PHIK/?tag=serieats-20

which measures 10x14 inches.

Alternatively, you can do as Catherine Lamb (and originally Kenji) suggest and use two 8x8" pans.