First of all, and not meaning to be the least bit unkind, we’d like to explain to our American readers that this question came from a cook in Britain, where it is perfectly acceptable to serve a fruit cake at a wedding, and where it does not cause the guests to giggle excessively, break out in a cold sweat, or consider the marriage doomed from Day One.
Second, while math is admittedly not our strong point, we believe you would have to quadruple a recipe for a 6-inch cake to fill a 12-inch cake pan.
Finally, assuming that you’re still reading, one of your own British publishers offers the best reference on producing fruit cakes of various strengths and sizes. The Ultimate Cake Decorator (Canada, UK) by Janice Murfitt and Louise Pickford has ingredients charts for making either a light fruit cake or rich fruit cake in any size from 6 inches to 13 inches.
For the 12-inch-square, light fruit cake (and the word light is used here only for the sake of comparison), use 12 cups of mixed dried fruit, 1 cup of mixed candied peel, 1-1/2 cups chopped ginger, 2 cups dried apricots, 8 teaspoons orange rind, 5 tablespoons orange juice, 5-1/2 tablespoons sherry, 9 cups of flour, 8 teaspoons of mixed spice (which we call apple pie spice), 6-1/3 cups of light brown sugar, 4-1/2 cups butter, and 11 eggs. The cooking time for this monster is 5 to 5-1/2 hours at 275°F (140°C or Gas level 1).
The rich cake has more fruit (5-1/4 cups of raisins, 4-1/2 cups of sultanas, 4 cups of currants, 2-1/4 cups of glacéed cherries, as well as the candied peels and zest), 3-1/3 cups of ground almonds in place of 3 cups of the flour, almost 3 cups of almond slivers, too, a little molasses, a bit less sugar, butter, and eggs, and no ginger.