You don't mean resizable, right?
What you want is auto change its size following by the window's size changed, if so, you can try set width and height with percent or you should write some lines in window.onresize function and resize modalpanel.
yes, you are right. I can do what you said. But it is very repetitive for every modalPanel. the RF can auto-centered the modalpanel, i think it maybe auto resized too. Or we can do it with jQuery in a simple way.