An image forming apparatus which is capable of executing real-size preview of image data which does not have output sheet size information or resolution information. The image forming apparatus stores a plurality of data items therein. A printer section prints a data item selected from the stored data items. A controller unit determines whether or not the selected data item has sheet size information. When it is determined that the selected data item does not have sheet size information, the controller unit prompts the user to select a sheet size for printing of the selected data item via a console section. An image for real-size preview is generated from the selected data item based on a sheet size selected via the console section, and displayed on the console section.