If you routinely specify a custom page size, you can add it to the Page Size menu in the New Document dialog. You add custom page sizes to the Page Size menu by editing the New Doc Sizes text file in the Presets folder (in the InDesign application folder). This text file is self-documenting; to define custom page sizes, open the file in a text editor and follow the instructions in the file itself.
Page sizes defined in the New Doc Sizes file will appear in the New Document dialog box and the Document Settings dialog box.