I only have an 8x8 or a 9” round

It really depends on your preference if you want to make less (8x8" or 9" round) or more (9x13").

The former two sizes are about the same volume, you would just scale down the recipe by 25%. For the latter you'd scale up the recipe +50%. This is a great example of the utter wretchedness of Imperial volumetric measurements and the superiority of mass-based metric measurements.

Otherwise, I'd do as Nicole suggests and bake off the extra batter in muffins or a suitably sized smaller mold. A muffin-size portion would be perfect for a brown bag meal or as a very presentable gift.

Best of luck.

Hi! I use this as a guide for sizing up or down: https://food52.com/blog/13239-how-to-make-your-baking-recipe-fit-your-pan-size

The 9x9 the recipe calls for is 81 square inches.
Your 8x8 is 64 square inches and a 9x13 is 117 sq. in.
So there's no easy transfer. I would suggest going with the 8x8 you have, and baking the excess batter in cupcake molds or something similarly small.

I am curious what everyone else on the hotline thinks!