The stencils were made using WOF's large armor templates and if I'm not mistaking, on his armor template thread he mentions about the percentage needed to achieve the size differences. You can unzip the armor parts and with an image editor you can resize each page (you might try to reduce the file size to 95% then increase/decrese it with 1% to your need). Always print out one page and compare it with your armor part, then if needed, resize the file and print it again for a quick compare. Do this until you have a perfect fit.
=====UPDATE=====
The exact percentage is 4%, which means that you need to reduce the stencil files to 96% to fit the medium size templates.