The physical size of a picture is controlled by a single number called DPI. There is no direct way of changing it in the application, because it has virtually no effect except telling the world what size is the picture supposed to be. The program that is then used for printing may or may not take this into account. I would recommend simply setting the required physical size in the program you use for printing.
The printing plug-in that allows you to set physical size before printing is not compatible with the current version of RW Paint. If you would want to use it, you would need to wait few weeks for the next version of Paint.
