The recipe doesn’t specify a particular size for the baking dish. However, a standard 9×13 inch dish is commonly used for casseroles and would work well for evenly spreading the croissant pieces and berries, ensuring a consistent bake. If you have a different size, just keep an eye on the casserole as the baking time might change slightly. The key is to make sure the ingredients fill the dish without overflowing and that the egg mixture soaks through the croissants properly.