using different size pans

If you are using a larger pan for the recipe, wouldn't you also have to increase the ingredients? I do't remember the geometry to figure the volume of a circle, but it sounds like you are doubling the pan volume size.

If/once you have that figured out, if the thickness (depth) of the cake batter is similar, then I would guess that the cooking time would be pretty much the same. If denser, in a heaver pan, etc., you may need to increase the cooking time. But for a cake, really, instead of the math, keep checking with the toothpick-comes-out-clean method to see if it's ready.

Anyone else with more experience, please chime in.

I now have more experience. I did the math and increased the ingredients. The depth of the batter in the 18x12 pan was the same as the original recipe in the 8 inch round. It worked fine. No need to change the temperature. I did monitor it. Used the toothpick method and it took a little longer then the 8 inch round (I was expecting that).

Accidently broke one of the layers and had to make an extra batch. Use the broken cake as a sampler. It turned out great. Now have two 2-layer slab cakes for a wedding this evening!