Pop up size not only in pixels but also in percentage

In the properties of a form you can define the width and height in pixels. I would like to be able to change the pixels in to a percentage. The viewports of the enduser differ so much that it is not always either to small or to big and the automatic function does not always give the desired result. The percentage option would give me much more control on how large the popup will be for the end user.

Regards,

Ronald