IE11 setting form height and width with javascript fails

Hi, i am testing my applications because my company will upgrade from IE8 to IE11. I have this javascript in my code.

if (document.forms['form'].windowWidth) { document.forms['form'].windowWidth.value = document.documentElement.clientWidth; }
if (document.forms['form'].windowHeight) { document.forms['form'].windowHeight.value = document.documentElement.clientHeight; }

It sets the width and height of the form to the w&h of the window. In IE8 it always worked. In IE11 on Windows7 it does not work. In IE11 on Win8.1 itsometimes work. When i debug the javascript it looks fine, values are filled. But nothing happens, the form does not resize in Win7/IE11. Anyone experiencing similar or same problem ? Thanks.

/Danny