How do you convert a recipe that fits into a 9 inch tart pan to a 11 inch tart pan?

If you are making a recipe that is for an 8"x8" square pan, and you want to use a 12"x12" pan instead, you should double your recipe (because the surface area of the 8" one is 64 square inches, and the surface are of the 12" pan is 144 inches, which is roughly double, although your cake may be a little thinner because it's a bit more than double).

If you're talking about round pans... the area of an 8" round pan is 50.2 square inches and the area of a 12" round pan is 113 square inches. A bit more than double. But I'd probably just double it as well.