An image forming apparatus capable of printing a non-standard size image on a sheet of a size as desired by the user as possible, and preventing stoppage of printing the image even when there is no sheet of the size desired by the user in a sheet feeder. When image data is for a non-standard size, if there is a sheet feeder accommodating recording sheets of which vertical and lateral lengths corresponding to vertical and lateral lengths represented by the image data are designated, a control unit feeds the sheets from the sheet feeding cassette. If there is no such a sheet feeder, the control unit feeds recording sheets of a free size from a manual feed tray accommodating the recording sheets of the free size.