If you don't mind a square cake instead of a round one, you can always convert a larger to a smaller one by making a "pan" the size you want with parchment or foil, then hold it in place with more foil a smaller pan, beans, pie weights or anything that will keep the shape while the cake bakes. I used to do this a lot when I first got married. All you need is the batter cup conversion. To help with that I hope the chart below helps:
EXPORTED FROM LIVING COOKBOOK
CAKE PAN SIZE CONVERSIONS
RECIPE CALLS FOR VOLUME USE INSTEAD:
1 (8 inch) round cake pan 6 CUPS
1 (8x4) inch loaf pan
1 (9 inch) round cake pan
1 (9 inch) pie plate
1 (11x7 inch) baking dish
2 (8 inch) round cake pans 12 CUPS
2 (8x4 inch) loaf pans
1 (9 inch) tube pan
2 (9 inch) round cake pans
1 (10 inch) Bundt pan
2 (11x7 inch) baking dishes
1 (10 inch) springform pan
1 (9 inch) round cake pan 6 CUPS
1 (8 inch) round cake pan
1 (8x4 inch) loaf pan
1 (11x7 inch) baking dish
2 (9 inch) round cake pans 12 CUPS
2 (8x4 inch) loaf pans
1 (9 inch) tube pan
2 (8 inch) round cake pans
1 (10 inch) Bundt pan
2 (11x7 inch) baking dishes
1 (10 inch) springform pan
1 (10 inch) round cake pan 11 CUPS
2 (8 inch) round cake pans
1 (9 inch) tube pan
1 (10 inch) springform pan
2 (10 inch) round cake pans 22 CUPS
4 (8 inch) round cake pans
3 or 4 (9 inch) round cake pans
2 (10 inch) springform pans
9 inch tube pan 12 CUPS
2 (9 inch) round cake pans
2 (8 inch) round cake pans
1 (10 inch) Bundt pan
1 (9 inch) tube pan
10 inch tube pan 16 CUPS
3 (9 inch) round cake pans
2 (10 inch) pie plates
2 (9 inch) deep dish pie plates
4 (8 inch) pie plates
2 (9x5 inch) loaf pans
2 (8 inch) square baking dishes
2 (9 inch) square baking dishes
10 inch Bundt pan 12 CUPS
1 (9x13 inch) baking dish
2 (9 inch) round cake pans
2 (8 inch) round cake pans
1 (9 inch) tube pan
2 (11x7 inch) baking dishes
1 (10 inch) springform pan
11x7x2 inch baking dish 6 CUPS
1 (8 inch) square baking dish
1 (9 inch) square baking dish
1 (9 inch) round cake pan
9X13X2 INCH BAKING DISH 15 CUPS
1 (10 inch) Bundt cake pan
2 (9 inch) round cake pans
2 (8 inch) round cake pans
1 (10x15 inch) jellyroll pan
10X15X1 INCH JELLYROLL PAN 15 CUPS
1 (10 inch) Bundt pan
2 (9 inch) round cake pans
2 (8 inch) round cake pans
1 (9x13 inch) baking dish
9X5 INCH LOAF PAN 8 CUPS
1 (9x2 inch) deep dish pie plate
1 (10 inch) pie plate
1 (8 inch) square baking dish
1 (9 inch) square baking dish
8X4 INCH LOAF PAN 6 CUPS
1 (8 inch) round cake pan
1 (11x7 inch) baking dish
8X1 1/4 INCH PIE PLATE 4 CUPS
1 (8 inch) round cake pan
9 X1 1/4 INCH PIE PLATE 5 CUPS
1 (8 inch) round cake pan
1 (9 inch) round cake pan
9X2 INCH PIE PLATE (DEEP DISH) 8 CUPS
2 (8 inch) pie plates
1 (10 inch) pie plate
1 (8 inch) square baking dish
1 (9 inch) square baking dish
1 (9x5 inch) loaf pan
10X1 1/2 INCH PIE PLATE 8 CUPS 2 (8 INCH) PIE PLATES
1 (8 inch) square baking dish
1 (9 inch) square baking dish
1 (9x5 inch) loaf pan
1 (9 inch) deep dish pie plate
9 INCH SPRINGFORM PAN 10 CUPS
1 (10 inch) round cake pan
1 (10 inch) springform pan
2 (8 inch) round cake pans
2 (9 inch) round cake pans
10 INCH SPRINGFORM PAN 12 CUPS
2 (8x4 inch) loaf pans
1 (9 inch) tube pan
2 (9 inch) round cake pans
1 (10 inch) Bundt pan
2 (11x7 inch) baking dishes
2 (8 inch) round cake pans
8 INCH SQUARE BAKING DISH 8 CUPS
1 (11x7) inch baking dish
1 (9x2 inch) deep dish pie plate
1 (9x5 inch) loaf pan
2 (8 inch) pie plates
9 INCH SQUARE BAKING DISH 8 CUPS
1 (11x7 inch) baking dish
1 (9x2 inch) deep dish pie plate
1 (9x5 inch) loaf pan
2 (8 inch) pie plates
If you have an unusual pan size, and would like to find out the capacity, measure the amount of water it takes to fill the pan. Then compare it with the pans above to determine the substitution. For your cake to rise evenly, you should only fill this pan to the half-way mark with batter. The baking time may change as well, so it is imperative that you keep a watchful eye on your cake, and check for doneness using your preferred method.
You also might find
this chart useful.