How we resize the popup in asp.net

Question

I m using a gridview in which i am using a linkbutton named assign in it.As i click on the linkbutton,a popup is opened but it is now resizable according to the controls used in it.So,I want to know that how i resize my popup window according to the controls used in it.I am using.......
<a href="javascript:openPopup('assign.aspx?id=<%# Eval("id")">