What's the easiest way?

. . . to make a page element to the right of a form that will contract or expand if someone increases the size of text-based elements on their screen?

I have a visual example.

Here is the actual page: http://innovyse.com/dev/order/index2.html

Here is the box-like element I want to add in the position I would like for it to be: http://innovyse.com/dev/img/width-a.gif

Here is how I would like that element to behave should the user increase text-size: http://innovyse.com/dev/img/width-b.gif

It seems to me this is simple, yet my mind is a blank.

Thank you,
David