Recipe says to use an 8 by 8 pan, but....?

Go to the web page, and read it, it has a table, and I can not paste it properly

Cake Pan Size Conversions

http://allrecipes.com/HowTo/Cake-Pan-Size-Conversi...

Yes, go ahead and double the recipe for the 13 x 9 pan.

Is your recipe doubling friendly? If so then, yes, double it. If not then your end product will not be as high and thick as it would be in an 8X8 pan. Either way you may want to adjust the baking temperature too so the middle will cook before the edges have a chance to burn or dry out. Maybe just 5 degrees cooler?

If you do not double the recipe and still use the 9X13 then you may want to lessen the baking time so your product doesn't overcook.

Yeah, double the recipe, but only add a few (like 5-10) minutes onto cooking time. Just keep an eye on it when it's in the oven.

You can use the 13x9, but the thickness of whatever your cooking will be thinner, so it won't take as long to cook. Keep an eye on it in the oven so it doesn't burn, but it will work fine. Just check it often.