For the 10” skillet, is that a skillet that measures 10” on bottom or across the top? Since it is a cake, I want to get the right size!

Agree with you.
By volume measurements, 12 inch skillet is more than 1/3 bigger than the 10 inch one.
By experience, it holds a double recipe of 9" pie, no problem.

I have a cast iron skillet - measures 12" on top and 10" on bottom. The author of the recipe answered a question and said 10" cake pan would work. I want to use my skillet but am trying to figure out if the 12" is too large. It measures the same on bottom as the 10" cake pan.

@Hillary with 2 L
If you have both a 10 in cake pan & 12 in skillet, fill the first with water and pour into the second. That will show you approx volume relation.
I estimate the water will fill about 2/3 the skillet...if you bake the original recipe in the bigger skillet, the cake will be wider and shallower.
It will need to bake shorter time, to avoid drying out or burning.
Or make 1 and 1/3 batch of recipe to go in 12 in skillet.
Do you want to go ahead or not?
Your decision & good luck & please tell us your results.

My skillet (a standard issue Lodge) is 9 15/16 at the top, 8 3/4 at the bottom; roughly equivalent to a 9 3'8" straight sided pan.

Hey there Smaug - I have several Calphalon stock pots, and the glass (!) lids also fit my various All Clad skillets. If you have stockpot(s), try the lids on your skillets and see if they work!

Thanks, SMSF (pronunciation?)- I use a 10" Calphalon lid and an old 8" Revereware lid on my two pans- they both have a curved outer rim that extends an extra 1/2" or so and curves back in to a piece that (supposedly) fits tightly inside the rim of the pan- they work adequately on the All Clad pans, but they're not what you'd call a close fit aand they rattle around quite a bit. just seems they could have made the pans a normal size, or at least made lids to fit them. I love that Calphalon lid- I almost never use the pan it came with, but I've used it regularly with probably 8-10 other pans.