Scaling at 100% on a printer doesn't mean real size, it should mean 1 pixel => 1 dot (related dpi). In practice, it is very difficult to control the entire sequence to print something at its real size (see this
page). I would say for Weasis it is impossible because it works on multi-systems and multi-printers.
If you calibrate the screen in the weasis preferences, you can display the image at the true size (new button appears in zoom).