The CustomPage.ini can be updated using notepad. Here is some info on how to add additional sizes.

 In the Notepad document, you need to add the size or sizes of prints that you would like to add to your Darkroom software. In the text document, type in the size of the print you wish to add, then an equal sign, then, the pixel measurements of the print size. For example, if you wanted to add a 9×13 print size to the list, you would type: 9×13 = 2700, 3900 -By indicating the print size, in this example 9×13, on the left side of the = sign, you are indicating what should be displayed in the Darkroom application. In this example, 9×13 will be displayed on the list of available print sizes. -The two integers that follow the = sign represent the width and height in pixels. In the above example, width = 2700 and height = 3900. Always indicate the width first, then the height. -To calculate the integer values, multiply the number of inches by 300. If you were adding a 9×13 print, you would do the following multiplication to discover the width and height: 9 * 300 = 2700 ; 13 * 300 = 3900 You can do this with any print size, for example a 5×7 print would be entered into the Notepad document as the following: 5×7 = 1500, 2100 To discover the integer values, do the following multiplication: 5 * 300 = 1500 ; 7 * 300 = 2100